Padmini Ekadashi is a unique and rare occurrence, taking place only during the Shukla Paksha of the Adhika (leap) month, which is also known as the Purushottama Masa. Worshipping Lord Hari during this month and on this specific day yields thousands of times more spiritual benefit than any ordinary day.
The legend was narrated by Sage Pulastya to Sage Narada, and later by Lord Krishna to Yudhisthira. In Kartavirya Yuga, there was a powerful King named Kritavirya who had many beautiful wives, but none of them could bear a son. Grieved by the lack of an heir to the throne, the king and his main queen, Padmini, decided to perform severe penance in the forest.
They performed austerities for thousands of years, but to no avail. Queen Padmini then approached the chaste Anasuya Devi, wife of Sage Atri, and asked for a solution. Anasuya Devi explained that the leap month is the most sacred of all months, and observing the upcoming Padmini Ekadashi during this month would surely please the Supreme Lord.
Queen Padmini followed the instructions, fasting completely and staying awake in devotional prayer. Pleased by her pure devotion, Lord Vishnu appeared and blessed her with a boon. She requested a son who would be unmatched in strength and could only be defeated by the Supreme Lord Himself. Thus, the glorious Kartaviryarjuna was born, demonstrating that Padmini Ekadashi holds the power to fulfill the most impossible desires.