Ekadashi fast - The Story of its Origin

“One should observe the Ekadashi fast because of the following story: Once, God was sleeping with his ten indriyas and mind – the eleventh indriya – drawn inwards. At that time, Mur Danav, the son of Nadijangh, came to do battle with God. But then, a young woman was created from the divine light of God’s eleven indriyas. Seeing her, Mur Danav proposed to her, ‘Please marry me.’ The young woman replied, ‘I have taken a vow that I will only marry the person who defeats me in a duel.’ Thus, a duel between Mur Danav and the young woman ensued, in which the young woman severed Mur Danav’s head with a sword. God was pleased with her and said, ‘Ask for a boon.’ The young woman requested, ‘On my day of observance, no one should eat grains. Furthermore, since I was born from the divine light of your eleven indriyas, my name is Ekadashi. Since I am an ascetic, on my day of observance, no one should indulge in any of the vishays related to the eleven indriyas, which includes the mind.’ Hearing Ekadashi’s wish, God granted her that boon. This is the story as it is narrated in the Purans.”

Source: BAPS