The Ekadasi fasting named Padmini Ekadashi happens only when there is an Adhika Masa or Extra month in a Hindu Lunar Calendar. There is an interesting associated with Padmini Ekadasi. This Katha was narrated by Pulastya Muni to Sage Narada.
In the Treta Yuga there lived a King named Krtavirya. He was a very powerful and famous king. But the king had no son to inherit the kingdom and the King was worried about the future of his people after his death. So he decided to go to forest and do tapas and other austerities to please Lord Vishnu.
Krtavirya’s wife, Padmini, came to know about her husbands intentions and decided to go along with him.
The king made an ashram near Sage Atri and started performing austerities but he was not able to please Lord Vishnu. A worried Padmini then consulted Sage Atri’s wife Anasuya. She advised her to observe two ekadasis falling in the Adhika Masa.
Krtavirya and Padmini then observed the two Ekadasis falling in Adhika Masa. Soon Lord Vishnu appeared before the couple and said that their wishes will be fulfilled as they had observed the two Ekadasis in Adhika Masa which is highly auspicious. (Please note that the Adhika Masa is dedicated to Lord Krishna and is also known as Purushottam Masa).
Queen Padmini in due course of time gave birth to Kartaviryarjuna, who was a very powerful king and he is even believed to have defeated and imprisoned Ravana.