Pandava-nirjala Ekadasi.
Giriraj Swami: Ekadasi is one of the regular celebrations in the Vaisnava calendar. It is observed eleven days after the full moon and eleven days after the new moon of every month. Even in the thirteenth, or leap month, called adhi-masa or purusottama-masa, which comes every three years, during which no other festivals are celebrated, Ekadasi is observed. Ekadasi is known as “the day of Lord Hari” and is said to be “the mother of devotion.” Of the sixty-four items of devotional service listed in Srila Rupa Gosvami"™s Bhakti-rasamrta-sindhu, keeping the fast on Ekadasi is one. In fact, observing Ekadasi is one of the first ten basic principles.