Safala Ekadashi take place on the month of Pousha ( December – January ). The main deity of the Safala Ekdashi is lord Bishnu ( Narayan ). It is mentioned at Brahmanda Purana, fasting on Saphala Ekadashi devotee becomes the dearest of all to to the almighty God.
The story behind Safala Ekdashi are narrated here.
All the normal rules are associated to this Ekadashi.