Aja Ekadashi falls in waning phase of the moon in the month of Bhadrapada (August – September) according to Hindi Hindu Calendar. In 2015, the date of Aja Ekadasi fasting is September 8. The importance of this Ekadasi was mentioned by Lord Krishna to Yudhisthira and is found in the Brahmavaivarta Purana. It is believed that a person who observes this Ekadashi becomes free from the reactions of his/her sinful activities.
Legend has it that the great King Harishchandra got back his kingdom, wife and dead son after performing the Ananda or Aja Ekadashi. It was Gautama Muni who asked Harishchandra to observe the Annada Ekadasi so that he will be able to overcome all the difficulties in life. The king got back all that he had lost and was able to rule his kingdom without any difficulties.
It is said that whoever fasts on this Ekadasi is surely freed from all the sins. There are no separate rules for fasting on the day. Rice and grains are avoided on the day.
Source – http://www.hindu-blog.com/2008/08/aja-ekadasi-or-ananda-ekadashi.html