The vibrant and colorful harvest festival of Onam is celebrated in the beginning of the first month of Malayalam Calendar (Kolla Varsham) called Chingam. Onam falls, on the 12th day of the waxing moon in the six month of Hindu calendar called Bhadon or Bhadrapada which is usually around August or September according to Gregorian calendar once the golden yellow rice fields have been harvested at that time of the year. There is no specific god associated with Onam. Neither is any special puja or ritualistic prayer ceremony which is performed. The celebrations of Onam start ten days before the big day. As it is celebrated during the harvest season, the beautiful state of Kerala can be seen in its magnificent best. Weather is pleasantly sunny and warm calling for mirth and celebrations. Fields look brilliant as they glow with golden paddy grains. It is also the boom time of fruits and flowers. After the month of deprivation, Karkidakam (last month of Malayalam Calendar), farmers are happy with a bountiful harvest and celebrate the festival to the hilt.
The Onam festival continues for ten days. The starting day is called the Thiru Onam. The festival commences from lunar asterism (a cluster of stars smaller than a constellation) Atham (Hastha) that appears ten days before asterism Onam or Thiru Onam. Atham is regarded as auspicious and holy day by people of Kerala. Onam corresponds to Shravan day in the month of August or September, hence it is also called Sravanotsavam. The first day of the festival is called Atham and culminating on Thiru Onam. These are considered to be the most important days. The day of Atham is celebrated on the basis of the position of the stars. The day is decided in accordance with the legend of King Mahabali in whose honor Onam is celebrated. People believe that it was this particular day in the month of Chingam when Lord Vishnu took his fifth avatar as Vaamana, appeared in the kingdom of King Mahabali and send him to nether world. It is believed that Onam celebrations started during the Sangam Period. Record of celebrations can be found from the time of Kulasekhara Perumals (800 AD). At that time Onam celebrations continued for a month.