This Saturday is Pandava Nirjala Ekadasi, an auspicious day within the Vaisnava calendar where we Vaisnavas fast from everything, including water, from sunrise to sunrise.  I'm often asked about Ekadasi, especially Pandava Nirjala Ekadasi, so the following is a short outline for those who don't know.  This will be quite long so do be patient.

This is an excerpt describing the phenomena of Ekadasi from the Padma Purana, "Kriya-sagara-sara".

Once the great sage Jaimini Rsi said to his spiritual master Srila Vyasadeva, "Oh Gurudeva! Previously, by your mercy, you described to me the history of the glories of the Ganga river, the benefits of worshiping Visnu, the giving of grains in charity, the giving of water in charity, and the magnanimity of drinking the water that has washed the feet of the brahmanas. O best of the sages, Sri Gurudeva, now, with great enthusiasm, I desire to hear the benefits of fasting on Ekadasi and the appearance of Ekadasi.

kasmad ekadasi jata tasyah ko va vidhir dvija kada va kriyate kim va phalam kim va vadasva me ka va pujyatama tatra devata sad-gunarnava akurvatah syat ko dosa etan me vaktum arhasi

"Oh Gurudeva! When did Ekadasi take birth and from whom did she appear? What are the rules of fasting on the Ekadasi? Please describe the benefits of following this vow and when it should be followed. Who is the utmost worshippable presiding deity of Sri Ekadasi? What are the faults in not following Ekadasi properly? Please bestow your mercy upon me and tell about these subjects, as you are the only personality able to do so."

Srila Vyasadeva, upon hearing this inquiry of Jaimini Rsi, became situated in transcendental bliss. "Oh brahmana sage Jaimini! The results of following Ekadasi can only be perfectly described by the Supreme Lord, Narayana, because Sri Narayana is the only personality capable of describing them in full. But I will give a very brief description in answer to your question."

"At the beginning of the material creation, the Supreme Lord created the moving and non-moving living entities within this world made of five gross material elements. Simultaneously, for the purpose of punishing the humans beings, He created a personality whose form was the embodiment of sin (Papapurusa). The different limbs of this personality were constructed of the various sinful activities. His head was made of the sin of murdering a brahmana, his two eyes were the form of the sin of drinking intoxicants, his mouth was made of the sin of stealing gold, his ears were the form of the sin of having illicit connection with the spiritual master's wife, his nose of the sin of killing one's wife, his arms the form of the sin of killing a cow, his neck made of the sin of stealing accumulated wealth, his chest of the sin of abortion, his lower chest of the sin of having sex with another's wife, his stomach of the sin of killing one's relatives, his navel of the sin of killing those who are dependent on him, his waist of the sin of self appraisal, his thighs of the sin of offending the guru, his genital of the sin of selling one's daughter, his buttocks of the sin of telling confidential matters, his feet of the sin of killing one's father, and his hair the form of lesser, less severe sinful activities. In this way, a horrible personality embodying all the sinful activities and vices was created. His bodily colour is black, and his eyes are yellow. He inflicts extreme misery upon sinful persons."

"The Supreme Personality of Godhead, Lord Visnu, upon seeing this personality of sin, began to think to Himself as follows: 'I am the creator of the miseries and happiness for the living entities. I am their master because I have created this personality of sin, who gives distress to all dishonest, deceitful, and sinful persons. Now I must create someone who will control this personality.' At this time Sri Bhagavan created the personality known as Yamaraja and the different hellish planetary systems. Those living entities who are very sinful will be sent after death to Yamaraja, who will in turn, according to their sins, send them to a hellish region to suffer.

"After these adjustments had been made, the Supreme Lord, who is the giver of distress and happiness to the living entities, went to the house of Yamaraja with the help of Garuda, the king of birds. When Yamaraja saw that Lord Visnu had arrived, he immediately washed His feet and made an offering unto Him. He then had Him sit upon a golden throne. The Supreme Lord, Visnu, became seated on the throne, whereupon He heard very loud crying sounds coming from the southern direction. He became surprised by this and thus inquired from Yamaraja, 'From where is this loud crying coming?'

"Yamaraja in reply said, 'Oh Deva! The different living entities of the earthly planetary systems have fallen into the hellish regions. They are suffering extremely for their misdeeds. The horrible crying is because of suffering from the inflictions of their past bad karma (actions).'

"After hearing this the Supreme Lord, Visnu, went to the hellish region to the south. When the inhabitants saw who had come they began to cry even louder. The heart of the Supreme Lord, Visnu, became filled with compassion. Lord Visnu thought to Himself, 'I have created all this progeny, and it is because of Me that they are suffering.'"

Srila Vyasadeva continued: "Oh Jaimini, just listen to what the Supreme Lord did next.

etac canyac ca viprarse
vicintya karunamayah
babhuva sahasa tatra
svayam ekadasi tithih

'After the merciful Supreme Lord thought over what He had previously considered, He suddenly manifested from His own form the deity of the lunar day Ekadasi.' Afterwards the different sinful living entities began to follow the vow of Ekadasi and were then elevated quickly to the abode of Vaikuntha. Oh my child Jaimini, therefore the lunar day of Ekadasi is the self-same form of the Supreme Lord, Visnu, and the Supersoul within the heart of the living entities. Sri Ekadasi is the utmost pious activity and is situated as the head among all vows.

"Following the ascension of Sri Ekadasi, that personality who is the form of the sinful activity gradually saw the influence she had. He approached Lord Visnu with doubts in his heart and began offering many prayers, whereupon Lord Visnu became very pleased and said, 'I have become very pleased by your nice offerings. What boon is it that you want?'

"The Papapurusa replied, 'I am your created progeny, and it is through me that you wanted distress given to the living entities who are very sinful. But now, by the influence of Sri Ekadasi, I have become all but destroyed. Oh Prabhu! After I die all of Your parts and parcels who have accepted material bodies will become liberated and therefore return to the abode of Vaikuntha. If this liberation of all living entities takes place, then who will carry on Your activities? There will be no one to enact the pastimes in the earthly planetary systems! Oh Kesava! If You want these eternal pastimes to carry on, then You please save me from the fear of Ekadasi. No type of pious activity can bind me. But Ekadasi only, being Your own manifested form, can impede me. Out of fear of Sri Ekadasi I have fled and taken shelter of men; animals; insects; hills; trees; moving and non-moving living entities; rivers; oceans; forests; heavenly, earthly, and hellish planetary systems; demigods; and the gandharvas. I cannot find a place where I can be free from fear of Sri Ekadasi. Oh my Master! I am a product of Your creation, so therefore very mercifully direct me to a place where I can reside fearlessly.'"

Srila Vyasadeva then said to Jaimini, "After saying this, the embodiment of all sinful activities (Papapurusa) fell down at the feet of the Supreme Lord, Visnu, who is the destroyer of all miseries, and began to cry.

"After this, Lord Visnu, observing the condition of the Papapurusa with laughter, began to speak thus: 'Oh Papapurusa! Rise up! Don't lament any longer. Just listen, and I'll tell you where you can stay on the auspicious lunar day of Ekadasi. On the day of Ekadasi, which is the benefactor of the three worlds, you can take shelter of foodstuff in the form of grains. There is no reason to worry about this any more, because My form as Sri Ekadasi Devi will no longer impede you.' After giving direction to the Papapurusa, the Supreme Lord, Visnu, disappeared and the Papapurusa returned to the performance of his own activities.

"Therefore those persons who are serious about the ultimate benefit for the soul will never eat grains on the Ekadasi tithi. According to the instructions of Lord Visnu, every kind of sinful activity that can be found in the material world takes its residence in this place of foodstuff (grain). Whoever follows Ekadasi is freed from all sins and never enters into hellish regions. If one doesn't follow Ekadasi because of illusion, he is still considered the utmost sinner. For every mouthful of grain that is eaten by a resident of the earthly region, one receives the effect of killing millions of brahmanas. It is definitely necessary that one give up eating grains on Ekadasi. I very strongly say again and again, 'On Ekadasi, don't eat grains, don't eat grains, don't eat grains!' Whether one be a ksatriya, vaisya, sudra, or of any family, he should follow the lunar day of Ekadasi. From this the perfection of varna and ashram will be attained. Especially since even if one by trickery follows Ekadasi, all of his sin become destroyed and he very easily attains the supreme goal, the abode of Vaikuntha."

OK, this explains quite well why we don't eat grains on Ekadasi but why fast from everything, including water, on Pandava Nirjala Ekadasi?  Well the name is a little bit of a hint but I'll provide the following to explain a little further (i.e. I'm cutting-and-pasting this next excerpt from sections of this site.  There are quite a few spelling errors but the narrative is not hindered by them).

Once Bhimasena, the younger brother of Maharaja Yudhisthira, asked the great sage Shrila VyAsadeva, the grandfather of the PANDavas, if it is possible to return to the spiritual world without having observed all the rules and regulations of the EkAdasii fasts.

Bhimasena then spoke as follows, “Oh greatly intelligent and learned grandfather, my brother Yudhisthira, my dear mother Kunti, and my beloved wife Draupadi, as well as Arjuna, Nakula and Sahadeva, fast completely on each EkAdasii and strictly follow all the rules, guidelines and regulative injunctions of that sacred day. Bring very religious, they always tell me that I should also fast on that day too. But, Oh learned grandfather, I tell them that I cannot live without eating, because as the son of VAyudeva – Samanaprana, (the digestive air) hunger is unbearable to me. I can give widely in charity and worship Lord Keshava properly with all manner of wonderful upacharas (items), but I cannot be asked to fast on EkAdasii. Please tell me how I can obtain the same merits result without fasting.”

Hearing these words, the grandsire of Bhima, Srila VyAsadeva said, “If you want to go to the heavenly planets and avoid the hellish planets, you should indeed observe a fast on both the light and dark EkAdasiis.”

Bhima replied, “Oh great saintly intelligent grandfather, please listen to my plea. Oh greatest of munis, since I cannot live if I eat only once in a day, how can I possibly live if I fast completely? Within my stomach burns a special fire named Vrika, the fire of digestion. Agni the fire-god, descends from Lord Vishnu through BrahmA, from BrahmA to Angirasa, from Angirasa to Brihaspathi, and from Brihaspathi to Samyu, who was Agni'’ father. He is the gatekeeper in charge of Nairritti, the south-eastern direction. He is one of the eight material elements, and Parikshit MahArAja, he is very expert at examining things. He examined MahArAja Shibi once by turning into a dove (for further information on this incident see Srila A.C. Bhaktivedanta Swami Prabhupada’s Srimad Bhagavatam commentary to 1:12:20. Purport.)

Agni is divided into three categories; DavAgni, the fire in wood, JatharAgni, the fire in the digestion in the stomach, and VadavAgni, the fire that creates fog when hot and cold streams mix in for example the ocean. Another name for the fire of digestion is Vrika. It is this powerful fire that resided in Bhima’s stomach.

Only when I eat to my full satisfaction does the fire in my stomach become satisfied. Oh great sage, I might possibly be able to fast only once, so I beg that you tell me of an EkAdasii that is worthy of my fasting and that includes all other EkAdasiis. I shall faithfully observe that fast and hopefully still become eligible for liberation’s release.”

Shrila VyAsadeva replied, Oh king, you have heard from me about the various kinds of occupational duties, such as elaborate Vedik ceremonies and pujas. In the Kali-yuga, however, no one will be able to observe all these occupational, functional duties properly. I shall therefore tell you how, at practically no expense, one can endure some small austerity and achieve the greatest benefit and resultant happiness. The essence of what is written in the Vedic literatures known as the PurANas is that one should not eat on either the dark or light fortnight EkAdasiis.” As stated in Srimad Bhagavatam (Mahabhagavat PurANam) 12:13:12 and 15.) the Bhagavat PurANam is itself the essence or cream of all VedAnta philosophy (sara-vedAnta-sAram), and the Srimad Bhagavatam’s unequivocal message is that of full surrender to Lord Sri Krishna and the rendering of loving devotional service to Him. Observing EkAdasii strictly is a great aid in that process, and here Shrila VyAsadeva is simply stressing to Bhima the importance of the EkAdasii vratam. “…One who fasts on EkAdasiis saved from going to the hellish planets.”

Hearing Shrila VyAsadeva’s words, the son of VAyu, Bhimasena, the strongest of all warriors, became frightened and began to shake like a leaf on a banyan tree in a strong wind. The frightened Bhimasena then said, “Oh grandfather, what should I do? I am completely unable and ill equipped to fast twice in a month throughout the year! Please tell me of the one fasting day that will bestow the greatest benefit upon me!”

VyAsadeva replied, “Without drinking even water, you should fast on the EkAdasii that occurs during the light fortnight of the month of Jyeshtha (May-June) when the sun travels in the sign of Taurus (Vrishabh) and Gemini (Mithun), According to learned personalities, on this day one may bathe and perform Achamana for pratiprokshana purification. But while performing Achamana one may drink only that amount of water equal to a drop of gold, or that amount it takes to immerse a single mustard seed. Only this amount of water should be placed in the right palm for sipping, which one should form to resemble a cow’s ear. If one drinks more water than this, he might as well have drunk wine – despite the soaring heat of summer (in the northern hemisphere and cold in the southern hemisphere).

“One must certainly not eat anything, for if he does so he breaks his fast. This rigid fast is in effect from sunrise on the EkAdasii day to sunrise on the DwAdashii day. If a person endeavours to observe this great fast very strictly, he easily achieves the result of observing all twenty-four other EkAdasii fasts throughout the entire year.

“On DwAdashii the devotee should bathe early in the morning. Then, according to the prescribed rules, guidelines and regulative injunctions, and of course depending on his ability, he should give some gold and water to worthy brAhmaNas. Finally, he should cheerfully honour prasAdam with a brAhmaNa.

“Oh Bhimasena, one who can fast on this special EkAdasii in this manner reaps the benefit of having fasted on every EkAdasii during the year. There is no doubt of this, nor should there be. Oh Bhima, now hear the specific merit one gets by fasting on this EkAdasii. The Supreme Lord Keshava, who holds a conch, discus, club and lotus, personally told me, ‘Everyone should take shelter of Me and follow My instructions.’ Then He told me that one who fasts on this EkAdasii, without taking even drinking water or eating, becomes free of all sinful reactions, and that one who observes the difficult nirjalA fast on Jyeshtha-shukla EkAdasii truly reaps the benefit of all other EkAdasii fasts.

“Oh Bhimasena, in the Kali-yuga, the age of quarrel and hypocrisy, when all the principles of the Vedas will have been destroyed or greatly minimised, and when there will be no proper charity or observance of the ancient Vedik principles and ceremonies, how will there be any means of purifying the self? But there is the opportunity to fast on EkAdasii and become free of all one’s past sins.

“Oh son of VAyu, what more can I say to you? You should not eat during the EkAdasiis that occur during the dark and light fortnights, and you should even give up drinking water (nir = no jalaa= water) on the particularly auspicious EkAdasii day of Jyeshtha-shukla EkAdasii. Oh Vrikodara (veracious eater), whoever fasts on this EkAdasii receives the merits of bathing in all the places of pilgrimage, giving all kinds of charities to worthy persons, and fasting on all the dark and light EkAdasiis throughout the year, in one go. Of this there is no doubt.

Oh tiger among men, whoever fasts on this EkAdasii truly becomes a great person and achieves all manner of opulence and wealth, grains, strength, and health. And at the fearful moment of death, the terrible YamadUtas, whose complexions are yellow and black and who brandish huge maces and twirl mystic pAsha ropes in the air for binding their victims, will refuse to approach him. Rather, such a faithful soul will at once be taken to the supreme abode of Lord Vishnu by the Vishnu-dUtas, whose transcendentally beautiful forms are clothed in gorgeous yellowish garments and who each hold a disk, club, conch and lotus in their four hands, resembling Lord Vishnu. It is to gain all these benefits that one should certainly fast on this very auspicious and important EkAdasii, even from water.”

When the other PANDavas heard about the benefits to be gained by following Jyeshtha-shukla EkAdasii, they resolved to observe it exactly as their grandfather Srila VyAsadeva had explained it to their brother, Bhimasena. All the PANDavas observed it by refraining from eating or drinking anything, and thus this day is also known as PANDava NirjalA DvAdashii (technically it is a Maha-DvAdashii).

Shrila VyAsadeva continued, Oh Bhimasen, therefore you should observe this important fast to remove all your past sinful reactions. You should pray to the Supreme Personality of Godhead, Lord Sri Krishna in this way making your sankalpa declaration, ‘Oh Lord of all the devas (demigods), Oh Supreme Personality of Godhead, today I shall observe EkAdasii without taking any water. Oh unlimited Anantadev, I shall break fast on the next day, DwAdashii.’ Thereafter, to remove all his sins, the devotee should honour this EkAdasii fast with full faith in the Lord and with full control over his senses. Whether his sins are equal in volume to Mount Sumeru or to MandarAchala Hill, if he or she observes this EkAdasii, the sins that have been accumulated all become nullified and are burned to ashes. Such is the great power of this EkAdasii.

“Oh best of human beings, although a person should also give water and cows in charity during this EkAdasii, if for some reason or other he cannot, then he should give a qualified brAhmaNa some cloth or a pot filled with water. Indeed, the merit achieved by giving water alone equals that gained by giving gold ten million times a day.

“Oh Bhima, Lord Sri Krishna has said that whoever observes this EkAdasii should take a Holy bath, give charity to a worthy person, chant the Lord’s Holy names on a japa-mAla, and perform some kind of recommended sacrifice, for by doing these things on this day one receives imperishable benefits. There is no need to perform any other kind of religious duty. Observance of this EkAdasii fast alone promotes one to the supreme abode of Sri Vishnu. Oh best of the Kurus, if one donates gold, cloth, or anything else on this day, the merit one obtains is imperishable.

“Remember, whosoever eats any grains on EkAdasii becomes contaminated by sin and verily eats only sin. In effect, he has already become a dog-eater, and after death he suffers a hellish existence. But he who observes this sacred Jyeshtha-shukla EkAdasii and gives something in charity certainly achieves liberation from the cycle of repeated birth and death and attains to the supreme abode. Observing this EkAdasii, which is merged with DwAdashii, frees one from the horrible sin of killing a brAhmaNa, drinking liquor and wine, becoming envious of one’s spiritual master and ignoring his instructions, and continually telling lies.

“Furthermore, Oh best of beings (Jivottama), any man or woman who observes this fast properly and worships the Supreme Lord JalshAyi (He who sleeps on the water), and who on the next day satisfies a qualified brAhmaNa with nice sweets and a donation of cows and money – such a person certainly pleases the Supreme Lord Vasudeva, so much so that one hundred previous generations in his family undoubtedly go to the Supreme Lord’s abode, even though they may have been very sinful, of bad character, and guilty of suicide, etc. Indeed, one who observes this amazing EkAdasii rides on a glorious celestial airplane (vimAna) to the Lord’s abode.

“One who on this day gives a brAhmaNa a waterpot, an umbrella, or shoes surely goes to the heavenly planets. Indeed, he who simply hears these glories also attains to the transcendental abode of the Supreme Lord, Shri Vishnu. Whoever performs the Shraddha ceremony to the forefathers on the dark-moon day called amAvasya, particularly if it occurs at the time of a solar eclipse undoubtedly achieves great merit, but this same merit is achieved by him who simply hears this sacred narration – so powerful and so dear to the Lord is this EkAdasii.

“One should clean his teeth properly and, without eating or drinking, observe this EkAdasii to please the Supreme Lord, Keshava. On the day after EkAdasii one should worship the Supreme Personality of Godhead in His form as Trivikrama by offering Him water, flowers, incense, and a brightly burning lamp. Then the devotee should pray from the heart, ‘Oh God of gods, Oh deliverer of everyone, Oh Hrishikesha, master of the senses, kindly bestow upon me the gift of liberation, though I can offer you nothing greater than this humble pot filled with water.’ Then the devotee should donate the waterpot to a brAhmaNa.

“Oh Bhimasena, after this EkAdasii fast and donating the recommended items according to his ability, the devotee should feed brAhmaNas and thereafter honour prasAdam silently.”

Shrila VyAsadeva concluded, “I strongly urge you to fast on this auspicious, purifying, sin-devouring DwAdashii in just the way I have outlined. Thus you will be completely freed of all sins and reach the supreme abode.”