The twice-monthly fast on the eleventh tithi of each paksha, sacred to Vishnu. There are 24 named Ekadashis a year, 26 in an Adhik Maas year.
Ekadashi, the eleventh day of each lunar fortnight, is Vishnu's tithi. Grain is set aside and the day is given to restraint, remembrance and lightness. Each of the year's 24 Ekadashis carries its own name and story: Nirjala (the hardest, waterless), Devshayani (Vishnu sleeps; Chaturmas begins), Prabodhini (he wakes; weddings resume), Vaikuntha Ekadashi in Margazhi, and the rare Padmini/Parama pair of Adhik Maas.
The fast is kept on the day Ekadashi prevails at sunrise (Smarta convention). When the tithi is 'lost' between two sunrises (Kshaya) the observance shifts by rule. This site's Ekadashi list resolves all of it per city.
No grains or pulses; fruit, milk and water by capacity. The fast is broken (parana) next morning within Dwadashi.