Ekadashi (Sanskrit: , ekādaśī, “eleven”), also spelled as Ekadasi, is the eleventh lunar day (Tithi) of the shukla (bright) or krishna (dark) paksha (fortnight) of every lunar month in the Hindu calendar (Panchang). In Hinduism and Jainism it is considered a spiritually beneficial day. Scriptures recommend observing an (ideally waterless) fast from sunset on the day beore Ekadashi to 48 minutes after sunrise on the day following Ekadashi.
Two Ekadashis occur in one month according to positions of the moon. One Ekadashi coincides with the times when the Moon is the closest to Earth (perigee) and the times when it is farthest from the Earth (apogee). These are also the times of high tide and low tide respectively. During Ekadashis in the Krishna paksha, the Moon is at its closest to the Earth and during Ekadashis in the Shukla paksha at its farthest from the Earth.
Bhagavata Purana (sk. IX, adhy. 4) notes the observation of Ekadashi by Ambarisha, a devotee of Vishnu.