Vamana Jayanti is dedicated to the Vamana Avatar who had appeared on the earth on this day. The Vamana Avatar is one of the Avatars that Lord Vishnu had taken to oust evil from the world.

He had taken ten incarnations out of which the Vamana Avatar is the fifth one. Vamana appeared in the Treta Yug. The Vamana Jayanti is observed on the dwadashi of the Shukla paksha of the Bhadrapada month as per the traditional Hindu calendar.

As per the Gregorian calendar, it falls on the 12th day of the waxing phase of the moon in the month of September.

King Bali was the grandson of the pious Prahlada. King Bali was an extremely brave king who kept performing his Ashwamedha Yajna. As per the scriptures after performing 100 Ashwamedha Yajna, the throne of Lord Indra could be claimed. He was so powerful that he defeated all the Devas. Alarmed, Indra sought the help of Lord Vishnu and requested for his intervention. However, Lord Vishnu told Indra that since King Bali was an extremely pious ruler, he will not kill him, but would devise some way to curb his growth.

Lord Vishnu then promised sage Kashyapa and his wife Aditi that he will be born to them. Duly Lord Vishnu is born to them and grows up to the age of 7 promptly. He had an aura around him and as he was short in his height, he was named Vamana. His father Sage Kashyapa performed the Ogudu ceremony where all the Gods gifted Vamana with variety of objects. For example, Lord Brihaspati gave him the Yajna Pavit or the sacred thread, Kubera  gifted him with a Golden water pot called Kamandalam, the Sun god preached him the Gayathri mantra and his mother Aditi gave him a small piece of red cloth or the Kaupinam.

Thus armed with gifts, Vamana reached where King Bali was performing his Ashwamedha Yajna. The mentor of the demons, Shukracharya recognized Lord Vishnu in his incarnation and advised King Bali not to give anything to the Vamana. But being the decent king that he was, King Bali opined that if it was Lord Vishnu, who had come to kill him, then it would be his honour to die in his hands or give up something for him. So King Bali asks what Vamana wanted. Vamana says that he wants three steps of land only which is measured by his foot. King Bali agrees and gives a Kamandalam. Shukracharya tries to stop this by becoming a bee and blocking the flow of the water. Vamana, with a dhurva grass pokes it away to clear the mouth of the Kamandalam. Shukracharya since then lost his one eye.

After receiving the dana or charity, Vamana grew gigantic in size and covered the world in one step and with his next step covered the other worlds and asked Bali where he should put his third step. Bali shows Vamana his own head. Thus Bali was subjugated and the worlds were conquered. Bali was sent to the Patala Loka. Lord Vamana pleased with Bali invites him to visit his kingdom once every year, which is celebrated as Onam in Kerala.

Lord Vamana has a temple in Thrikkakkara near Kochi where the day is observed with a lot of fanfare and devotees worshipping Lord Vishnu in the form of Lord Vamana. The original name of the place was Thiru-kal-kari meaning a place where the holy feet were placed.