Vaman Jayanthi is dedicated to the Vamana Avatar of Hindu God Vishnu. It is believed that Vaman (the dwarf) incarnation appeared on earth on Vaman Jayanti day. Vaman Jayanthi 2017 date is September 2 in some regions and in some it is on September 3. Annually it is observed on the 12th day of the Shukla Paksha of Bhadrapad month – 12th day of the waxing phase of moon.
Vamana appeared on earth to give moksha to Asura King Bali and also help Devas (Demi gods) who had lost their power to King Bali.
The legend of Vaman varies from region to region. In some regions, Bali is portrayed as an egoistic demon king and in some regions he is a kind-hearted king. What is common in all legends is that Indra and other Devas had lost their power to King Bali and takes refuge at Vishnu’s feet.
King Bali was an ardent devotee and was only happy to surrender before Vishnu and attain Moksha.
You can read more about Vaman Jayanti in this article.