Ekadashi is the eleventh tithi, and it is the most widely observed of all the lunar days. Occurring twice a month, in both the bright and dark fortnights, it is kept as a fast day sacred to Vishnu; devotees abstain especially from grains and beans from one sunrise to the next. In the fivefold grouping it is a Nanda tithi, and its presiding deities are traditionally the Vishvadevas, the universal gods. The tradition holds the Ekadashi fast to be among the most meritorious of regular observances, and particular Ekadashis through the year carry their own names and stories. The waxing Moon at the bright Ekadashi is nearly full. These are the devotional conventions of Vaishnava practice and the wider calendar, resting on the fixed count of eleven days from new or full moon.
The Vishvadevas, the "all-gods," are a Vedic category of minor deities personifying universal principles rather than a single named figure, and their assignment to Ekadashi reflects the tithi's own comprehensive, all-encompassing devotional weight in Vaishnava practice rather than pointing to any one myth. Because Ekadashi recurs every fortnight, the fast is one of the very few observances in the Hindu calendar kept twice monthly without exception, and different named Ekadashis through the year, Nirjala, Devshayani, Devutthana and many others, are distinguished from one another by the lunar month and fortnight in which they fall, each carrying its own associated story even though the underlying tithi and its Ekadashi fasting rule are identical every time.