Wikipedia
Ekadashi (, , , 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 and is usually observed by a partial fast. Beans and grains are not eaten during Ekadasi, as on this day these two foods are believed to be contaminated by sin. Only fruits, vegetables and milk products are eaten during Ekadasi. This period of abstention runs from sunrise on the day of Ekadasi to sunrise on the day following Ekadasi.
Two Ekadasis occur in one month according to positions of the moon. The progression of the moon from full moon to new moon is divided into fifteen equal arcs. Each arc measures one lunar day, called tithi: The time it takes the moon to traverse that distance is the length of that lunar day. Ekadashi refers to the 11 tithi, or lunar day. The eleventh tithi therefore corresponds to a precise phase of the waxing and waning moon: In the bright half of the lunar month, the moon will appear roughly 3/4 full on Ekadasi, and in the dark half of the lunar month, the moon will be about 3/4 dark on Ekadasi.
Bhagavata Purana (sk. IX, adhy. 4) notes the observation of Ekadashi by Ambarisha, a devotee of Vishnu.