A lunar day
Sanskrit tithi: one thirtieth of the moon's cycle, the time the moon takes to gain twelve degrees on the sun.
Festivals are fixed by tithi, not by the date on a solar calendar, which is why they move from year to year. A tithi can begin and end at any hour, so check a panchang for your place.
The Panchang, Hinduism's lunar-solar calendar, tracks the relationship between Sun and Moon across five limbs to determine auspicious timings and festivals.
From the Brahmavaivarta Purana, truthful King Harishchandra regains his kingdom, wife, and resurrected son by observing Aja Ekadashi.
King Mahidhwaja, murdered and trapped as a restless spirit, is freed when Sage Dhaumya dedicates his Apara Ekadashi merit to the king's liberation.
The divine katha of Harishayani Ekadashi marking the beginning of the four sacred months of Chaturmas when Lord Vishnu rests upon Sheshanaga.
A guide to Ekadashi Vrat: Ekadashi Devi's origin, the three modes of fasting, foods to avoid, puja vidhi, and the rules of Dwadashi Parana and Hari Vasara.
The Padma Purana katha of Lalit and Lalita, where a wife's Kamada Ekadashi fast and surrendered merit freed her husband from a demonic curse.