Significance and legend
This Ekadashi falls during Pitru Paksha, the fortnight of the ancestors. In the Brahmavaivarta Purana dialogue, Krishna tells Yudhishthira of King Indrasena of Mahishmati, a devotee of Vishnu who ruled with justice. One day the sage Narada arrived at his court with a message. During a visit to the realm of Yama, Narada had met Indrasena's late father, who, because he had broken an Ekadashi vow during his lifetime, was not able to reach the higher worlds. The father asked Narada to tell his son to observe Indira Ekadashi and offer its merit to him.
Narada described the procedure to the king: shraddha for the ancestors at midday, a fast through the day, worship of Shalagrama, and a vigil through the night. Indrasena observed the vrata with his family and servants. At its conclusion, flowers rained from the sky, and his father ascended to Vishnu's abode on Garuda. In time, Indrasena himself ruled long and attained heaven.
How Indira Ekadashi is observed
On Dashami the observer bathes and performs tarpana, eating a single meal. On Ekadashi, besides fasting, the observer performs shraddha with pinda offerings for the ancestors at midday, feeds Brahmins, and gives the leftover pinda to a cow. Vishnu, often as Shalagrama, is worshipped and the night is spent in vigil. On Dwadashi after sunrise, within the parana window, food is offered to Vishnu and the fast is broken in the company of family.