In the Brahma-vaivarta Purana it is said that one who observes fasting on Ekadasi day is freed from all kinds of reactions to sinful activities and advances in pious life.
Origin of Ekadasi
In the fourteenth chapter of
Padma Purana, in the section named
Kriya-sagara-sara, Srila Vyasadeva explains the origin of Ekadasi to Sage Jaimini as follows:
At the beginning of the material creation, the Supreme Lord, for the purpose of punishing the sinful human beings, created a personality whose form was the embodiment of sin (Papapurusha). The different limbs of this personality were constructed of the various sinful activities. In order to control Papapurusha, the personality known as Yamaraja came into existence along with the different hellish planetary systems. Those living entities that are very sinful are sent after death to Yamaraja, who will in turn, according to their sins, send them to a hellish region to suffer.
The living entities, according to their karmic activities thus began to enjoy or suffer. Seeing so many souls suffer in hellish condition, the compassionate Lord began to feel sorry for them. In order to help them He manifested from His own form the deity of the lunar day Ekadasi. Thus, Ekadasi is the personification of the vow to fast on the eleventh day of the lunar month. Therefore Ekadasi is the selfsame form of the Supreme Lord Vishnu. Sri Ekadasi is the utmost pious activity and is situated at the head among all vows.
Afterwards the different sinful living entities began to follow the vow of Ekadasi and were then elevated quickly to the abode of Vaikuntha. Following the ascension of Sri Ekadasi, Papapurusha (sin personified) gradually saw that his own existence was being threatened. He approached Lord Vishnu praying that, ‘O Lord, I am your created progeny, and it is through me that you wanted distress given to the living entities who are very sinful. But now, by the influence of Sri Ekadasi, I have become all but destroyed. You please save me from the fear of Ekadasi. No type of pious activity can bind me. But Ekadasi only, being Your own manifested form, can impede me. I cannot find a place where I can be free from fear of Sri Ekadasi. Oh my Master! I am a product of Your creation, so therefore very mercifully direct me to a place where I can reside
After this, Lord Vishnu, observing the condition of the Papapurusha began to speak thus: ‘Oh Papapurusha! Rise up! Don’t lament any longer. Just listen, and I’ll tell you where you can stay on the auspicious lunar day of Ekadasi. On the day of Ekadasi, which is the benefactor of the three worlds, you can take shelter of foodstuff in the form of grains. There is no reason to worry about this any more, because My form as Sri Ekadasi Devi will no longer impede you.’ After giving direction to the Papapurusha, the Supreme Lord Vishnu disappeared and the Papapurusha returned to the performance of his own activities. According to the instructions of Lord Vishnu, every kind of sinful activity that can be found in the material world takes its residence in this place of foodstuff (grains). Therefore those persons who are serious about the ultimate benefit for the soul will never eat grains on the Ekadasi day.
This is the story behind ekadashi vrat puja.
As we all know its importance is fasting on ekadashi may free you from sins. Different Ekadashi has different story behind it. Common is some sage suggests some x person to fast on x ekadashi to free from x sin.
1 Origin of Ekadashi