Parsva Ekadasi occurs in the month of Asvina when the Moon is waxing, and is also known as Vamana Ekadasi. In the Brahma-vaivarta Purana, Sri Krsna and Maharaja Yudhistira discuss the illimatable glories of this special day.
Maharaja Yudhistira asked Sri Krsna "What is the name of the Ekadasi that takes place during the waxing moon in the month of Asvina? How should one make a proper job of observing this Ekadasi vrata and what merit is to be obtained by it? Please explain to me all these in details."
Sri Krsna replied. "O King, the glories of this day are immeasurably great, it awards liberation and revives one's desire. This Ekadasi is also known as Vamana Ekadasi due to the Lords appearance as Vamana deva at this time. It is also known as Parsva Parivartini Ekadasi ("changing sides") Ekadasi."
Maharaja Yudhistira enquired, "O sweet Janardana! In spite of hearing your description I still have some doubts. O King how do you sleep and how do you change sides? What procedure can be carried out for fulfilling the Caturmasya Vrata. What should people do when you are sleeping? Why did you bind Bali Maharaj in your impenetrable way? This I am unable to understand? Why is this?
Sri Krsna replied: My dear Yudhistira, there was no reason for the appearance of Vamana deva in particular, nevertheless his appearance was most auspicious what with his being the Supreme Personality of Godhead. Your doubts will be removed in good time when you have finished hearing this description of Ekadasi and your mind is pacified by it's observance.
"O lion like King! In Treta-yuga there was a great devotee known as Bali Maharaj, he was born in a demoniac family, yet he still became a most sublime devotee. This is almost unheard of! By his regular worship of me by offering a whole variety of sweet and sincere prayers I became most satisfied. His benign service unto me was of a most high calibre and I became very pleased simply by hearing his words. He also worshipped many brahmanas and performed sacrifice. Soon he became so prominent he defeated Indra and, the King of heaven, and conquered the heavenly planet. Then Indra approached me and offered beautiful prayers in harmony with the desires of the personality of Godhead. By their prayer I assumed the form of a dwarf (Vamana) and went to the sacrificial arena of Bali in the garb of a brahmacari.
"Upon asking Bali for three steps of land I grew very ashamed of myself and only when I desired something more than these three steps did I feel at ease. Vamana deva thus proceeded to take three steps, the first step covered the lower planetary systems and the second extended beyond the limited confines of the material universe. The third step he happily placed on my head and I grew to be most ecstatic. Being fully satisfied by the Lords humility he was more than kind enough to do this and I blessed him by living constantly with him.
"On the day of this Ekadasi which occurs during the waxing moon of Asvina. A beautiful and most acutely aware deity of Vamana deva was installed in the residence of King Bali. Another form of mine has been established in the bed of Ananta Sesa in the ocean of milk. The Supreme Lord sleeps for four months beginning from Sayana Ekadasi up to Utthana Ekadasi. One should offer special worship to the Lord with selfish motivation. He should properly observe each and every Ekadasi and by observance in this way one obtains the result of performing one thousand horse sacrifices.