Primarily the rice harvest festival of Kerala, Onam, is also known as the festival of rain flowers. But for most of the Keralites, Onam is the homecoming day of their benevolent king Mahabali, who ruled Kerala long ago. Legend has it that King Mahabali was so sincere, honest, just and good that his citizens almost forgot the Devas and their pujas to them which made them jealous. So they requested Lord Vishnu to put an end to this situation. Lord Vishnu could not deny their request as it was a matter of keeping the whole system in balance. Therefore, in spite of King Mahabali being his devout worshipper, he tricked him using his only weakness, the Ego. Lord Vishnu took the form of Vamana Avatar and defeated him by pushing him down into the Patala Log. Yet, because he was such a perfect King, he was granted one boon – to visit his kingdom once every year on the day of Onam.

The festival is celebrated over a span of 10 days as it is believed that it took the king 10 days to travel from Patala log to Earth. The first day is the day he starts to travel back to visit his kingdom. On this day, the festival kicks off in Kochi, Kerala, with the Royal parade called Attchamayam leaving the temple dedicated to Lord Vamana at Thrikakkara, the ancient capital of his Kingdom. This parade depicts all the elements of Kerala culture, including royally caparisoned elephants and numerous floats and tableaux. Once the Kodiyettu (flag hoisting) in this temple is done, the Onam celebrations in other temples can commence. The temple at Thrikakkara is considered to be the centre point of Onam celebrations. Apart from the parade on the first day, two more grand processions named Pakalpooram and Seeveli are held in the temple on the last day (9th day) and on Onam day respectively. These two parades attract a lot of devotees and tourists. Along with the processions, Sadaya at the temple grounds is also an important attraction. Sadaya is a traditional feast prepared with seasonal vegetables and served on a banana leaf. This elaborate meal has a minimum of nine courses and can go up till two dozen dishes in some households.

Though declared a taboo (Haram) by the Islamic preachers, some Muslim communities celebrate Onam along with the Christian and Hindu Communities in a big display of communal harmony in the state. Therefore big and beautiful Pookolams (decorated arrangement of flowers on the floor) are made at the entrance of houses and temples, images of Onatthappan (Vamana) are put up at the house, the performances of Kummattikali (dance with wooden masks) are done at Thrissur, Kathakalli (traditional dance of Kerala) at Valluvanad. Cheruthuruthy and Thriruvathirakali (executed by a group of dancers in a circular pattern and by clapping hands around a lamp) are also performed. Pulikali (an enactment of tiger hunting done by dancers painted like tigers and hunters) is done on 4th day of Onam in Thrissur, Theyyam in the North Malabar region of Kerala and a showcase of percussion instruments like Panchavadhyam, Chenda etc, along with Vallamkali (Boat race) are also done at Aranmula. Onam Sadaya at almost all households, marks the celebration of the Onam festival in Kerala. Onam is celebrated all over Kerala, but Kochi, Trivandrum, Thiruvananthapuram, Thrissur, and Kottayam have the most vibrant celebrations.