According to the mythology it is said that once there was a king named Sukentumaan who ruled Bhadravati Puri, whose world was barren without a son even after many years of marriage. The king and the queen was extremely tensed and worried about their future, thinking about that who will give Tarpan and Pind Daan after their death and who will take the throne in the future. Due to frustration regarding this matter one day king went to the deep forest, after some time the king felt thirsty he went deep inside the forest in the search of water. He went in the direction of the sound of the animals and suddenly he found a lake. After drinking the water his eyes started to flutter and he understood that it was the indication of something nice to happen with him.
There he saw some rishi and munnis were chanting Shloka, he went there and bowed each of them with respect and devotion. He asked the purpose of the prayer, then the munni replied today is Ekadashi on which if a couple keeps a holy fast and pray the Lord Narayana (Vishnu) then his progeny will be take birth and would serve the purpose of the family forever. Listening to this the purpose of king was completed he kept the fast with full dedication and performed all the holy ritual with great dedication and love for his Lord Narayana. Later the queen conceived a baby, a cute little prince was born, became a mighty king in the future.
Fasting is not only the thing which has to be performed by the couple there are certain necessary rituals which is to be performed like one has to dedicate their mind completely to the Lord, couple has to be awake throughout the night and fast should be opened by the Ekadashi rule only.