Ekadashi is the eleventh lunar day of the waxing and waning lunar cycles in a Vedic calendar month. Ekadashi is popularly observed within Vaishnavism one of the major paths within Sanatana Dharma. From Wikipedia
Devotees prepare for fasting, prayer, and rituals as Drik Panchang confirms tithi and parana timings for this sacred day dedicated to Lord Vishnu.