Nirjalā Ekādaśī 2186

When the Pandavas asked about the Ekadashi fasts, Bhimasena confessed to his grandsire Vyasa that he could keep none of them. A fierce fire called Vrika burned in his stomach, and he could not go a single day without food, though his brothers, mother Kunti and Draupadi all fasted.

Vyasa then gave him one vow for the whole year: the Ekadashi of Jyeshtha Shukla, to be kept nirjala, without even a drop of water, from sunrise on Ekadashi to sunrise on Dvadashi. Only a sip for achamana is allowed. On Dvadashi one bathes, worships Vishnu, gives water and gifts to brahmanas, and then eats.

Vyasa promised that this single waterless fast grants the merit of all the Ekadashis of the year. Bhima kept it, and so it is lovingly called Bhimseni or Pandava Nirjala Ekadashi.