The legend or story and the importance of Putrada Ekadashi or Pavitropana Ekadasi vrata are mentioned in Bhavishyottara Purana as a discussion between Lord Sri Krishna and King Yudhistira. Putrada Ekadasi 2013 date is August 17.
When King Yudhistira asked about the Ekadasi Vrata which blesses the devotee with a good and healthy son and everlasting fame for many generations, Lord Krishna explained the story of King Mahijit who ruled Mahishmatipuri in the beginning of Dwapara Yuga.
King Mahijit was a successful and victorious king but he felt bad because he had no son. He performed many charities, good deeds and observed many homas, yagnas and yagas (sacrifices) but he has not find any result.
One day he went to Sage Lomasa, who was born from the hair of Lord Brahma, and explained his problem. Sage Lomasa went into meditation and told that the King Mahijit committed a sin in his previous birth which made him childless in this birth. King Mahijit was a vysya (merchant) in his previous birth.
When a cow and her newly born calf were about to drink water at a lake, he went there and drove them away for drinking water himself. Though, by his other good deeds he born as a King in his next birth, he became childless.
When Mahijit requested the Sage Lomasa for a remedy to pacify his mistake, he told about the Putrada Ekadashi or Pavitra Ekadasi vrata. King Mahijit observed Putrada Ekadashi vrata with great reverence and got a handsome and healthy son who ruled the country successfully after him.