Indira Ekadashi is observed on the Bhadrapada Krishna Paksha Ekadashi as per the lunar calendar.Devotees observe the Ekadashi Vrat and also read the Ekadashi Vrat Katha.
Ekadashi Vrat Katha
Once in Satya Yuga there was a city by the name of Mahishmita ruled by king Indrasena . He was a great king who had every thing a man could ask for, children, wealth, peoples love. He was also a great warrior and all his enemies were afraid of him. He ruled his kingdom with Dharma and treated the people of his kingdom as his own children.
Once sage Narada visited King Indrasena in his court. King invited the Sage with devotion. He worshipped the sage and gave him a suitable seat. Narada who was pleased with Indrasena blessed him. King Indrasena humbly asked Sage Narada the reason for visiting him.
Narada told that he once visited Yama in the Yamaloka or Hell where he had seen the father of Indrasena. Narada told that the father of Indrasena was a great scholar and also a great ruler. Not only that the father of Indrasena was also a great donor who used to donate all the wealth among the poor. Narada was astonishd to see him in the hell. Narada questioned him to tell the reason for he being in the hell or Yama Loka. Father of Indrasena told that he had once vowed to observe the Ekadashi Vrat during the Bhadrapada Krishna Paksha or Indira Ekadashi and did not observe the Vrat fully i.e. he abandoned the Vow in the middle, as a punishment of which he is in hell or Yama Loka.
Father of Indrasena requested Narada to tell his son(Indrasena) that if Indrasena observes the Ekadashi Vrat during the Bhadrapada Krishna Paksha Ekadashi or Indira Ekadashi he will be able to go to heaven.
Narada told Indrasena that he was there to convey the message of his father. Indrasena was happy and wanted to observe the Ekadashi Vrat but did not know the process. He prayed to sage Narada to kindly explain him the process of how to observe the Ekadashi Vrat on Bhadrapada Krishna Paksha Ekadashi or Indira Ekadashi.
Narada explained the process of Indira Ekadashi as follows:
Indira Ekadashi falls during the Pitru Paksha or Mahalaya Paksha. On the day of Indira Ekadashi one should get up early in the morning and take a head bath. Take a vow of observing the Indira Ekadashi Vrat in the morning. Perform the Shraddha to the Pitru devatas or the deceased ancestors with devotion. Fasting is done for the whole day of Indira Ekadashi. Head bath should be done in the evening also and fasting is observed in the night also. People who cannot observe a complete fast can eat fruits in the night. The one observing the fast should be awake the whole night reciting the name of Lord Vishnu or the slokas of Lord Vishnu. On the morning of the next day that is Dwadashi, food should be taken before the Dwadashi time gets over. It is advisable to feed a Brahmin before eating the food.
Narada after explaining the process left the court. Indrasena on the Bhadrapada Krishna Paksha Ekadashi observed the Indira Ekadashi Vrat as explained by Narada.
With the affect of Indrasena observing the Vrat, his fathers sins were forgiven and he went to heaven.
As the affect of the Indira Ekadashi Vrat is that it would help the deceased ancestors to go to heaven by forgiving their sins, this Ekadashi is called as Indira Ekadashi.