Onam commemorates Vamana (the fifth avatar of Hindu deity Vishnu) and the generous daitya king Mahabali. According to the Hindu legends, after Indra (the king of the devas) is defeated by Mahabali (the king of the asuras), the devas ultimately seek refuge in Vishnu, who agrees to restore Indra to power. To do so, Vishnu incarnates as a dwarf man called Vamana. The devoted Mahabali conducts ritual sacrifices, one of which is attended by Vamana, who requests only three feet (steps) of land to build a fire-altar. Mahabali agrees, despite being warned about Vamana’s true nature by the sage Shukra. Vamana grows in size, and in three strides, encompasses all of existence and beyond. The three worlds are restored to Indra, and Mahabali and the asuras are banished to the netherworld (Patala).However, witnessing Mahabali’s love for his subjects, Vamana grants the king’s sole wish to visit his kingdom once every year. This homecoming of Mahabali is celebrated as Onam in Kerala every year.
The date of Onam celebration is based on the Panchangam, and falls on the 22nd nakshatra Thiruvonam in the month Chingam of Malayalam calendar, which in Gregorian calendar falls between August–September.