Onam (Malayalam: ഓണം) is a State-wide Harvest festival with Hindu mythological background festival celebrated in Kerala, India. It is also the state festival of Kerala with State holidays on 4 days starting from Onam Eve (Uthradom) to the 3rd Onam Day. Onam Festival falls during the Malayalam month of Chingam (Aug - Sep) and marks the commemoration of Vamana avatara of Vishnu and the subsequent homecoming of mythical King Mahabali. Onam is reminiscent of Kerala's agrarian past, as it is considered to be a harvest festival.
The festival falls during the Malayalam month of Chingam (Aug - Sep) and marks the homecoming of the mythical King Mahabali who Malayalees consider as their King. In Kerala, it is the festival celebrated with most number of cultural elements such as Vallam Kali, Pulikkali, Pookkalam, Onatthappan, Thumbi Thullal, Onavillu, Kazhchakkula, Onapottan,[2] Atthachamayam etc.
Significance

        Onam is an ancient festival which still survives in modern times. Kerala's rice harvest festival and the Festival of Rain Flowers, which fall on the month of Chingam, celebrates the Asura King Mahabali's annual visit from Patala (the underworld). Onam is unique since Mahabali has been revered by the people of Kerala since prehistory.The King is so much attached to his kingdom that it is believed that he comes annually from the nether world to see his people living happily. It is in honour of King Mahabali that Onam is celebrated. The deity Vamana, also called Onatthappan,[3] is also revered during this time by installing a clay figure next to the floral carpet (Pookalam) .The birthday of Sri Padmanabhan, the presiding Deity of Thiruvananthapuram, is on the Thiruvonam day in the month of Chingam. Thiruonam day is the most important day of Onam. In Onam 2014 Thiruvonam date is September 7.

Rituls
Onam falls in the month of Chingam, which is the first month according to the Malayalam Calendar. The celebrations begin within a fortnight of the Malayalam New Year and go on for ten days. All over the state of Kerala, festive rituals, traditional cuisine, dance and music mark this harvest festival. The ten day Onam festival is considered to be flagged off with Atthachamayam (Royal Parade on Atham Day) in Thripunithara (a suburb of Kochi City).[7] The parade is colourful and depicts all the elements of Kerala culture with more than 50 floats and 100 tableaux. The main center of festival is at Vamanamoorthy Thrikkakara temple within Kochi City, believed to be the ancient capital of King Mahabali. The temple is dedicated to Lord Vamana and is directly linked to the mythological background of Onam.
          The ten days of Onam are celebrated with great fanfare, by Malayalees. Of all these days, most important ones are the first day, Atham, and the tenth and final day, Thiru-Onam (Thiruvonam). The rich cultural heritage of Kerala comes out in its best form and spirit during the festival.