Today is ekadasi. Not just any ekadasi, but nirjal-ekadasi: a total fast, even from water. This Pandava nirjal-ekadasi takes place at the hottest time, usually in June, in the high summer season.
Srila Sridhar Maharaj explains the history behind this ekadasi:
"Bhima could not control his hunger: his hunger was like a serious disease. So he approached Vedavyasa and said: 'There are many stories in the shastras in praise of observing ekadasi but it is not possible for me to fast. I can't tolerate hunger, so how can I observe Ekadasi? Please recommend something for me.'
"Srila Vedavyasa then advised: 'You must observe at least one ekadasi in the year, this Pandava nirjal-ekadasi. At this hottest time of the year, you shall not take even one drop of water.' So it was sanctioned like that. Bhima observed only one ekadasi, without a drop of water."
Of course, this concession was made for the Pandava, Bhima — also known by the epithet Vrikodarah, the voracious eater — in the Dvapara Yuga, but many devotees who have not strictly or properly followed ekadasi throughout the year observe this nirjal-ekadasi as a way of atoning for their transgressions.
Generally, though, such strict fasting is not recommended. In The Golden Staircase, Srila Guru Maharaj says:
"It is described in the shastras that in Satya Yuga, as long as a man's bones exist, that is how long he will live. Along with the longevity of the bones, the life will be there. In Treta Yuga life may be maintained by the nervous system. And it is stated that in Kali Yuga one's longevity depends on food.
"In Kali Yuga it is not possible to live without food. The only continuous fast allowed in Kali Yuga is for twenty-four hours: not more than that. Twenty-four hour fasting is the maximum allowed, because without food a man cannot survive."
Srila Bhakti Siddhanta Saraswati Prabhupad did not advocate unnecessary fasting either. His maxim was: "Take full prasadam, and do full seva." He preached: "Take whatever is necessary for the cause of Krishna, not for your own cause. You are Krishna's: if you grow weak, your service will be hampered, and you will be the loser."
So take full prasadam, and do full service!