Vibrant and colorful harvest festival of Onam is celebrated in the beginning of the first month of Malayalam Calendar (Kolla Varsham) called Chingam. This corresponds with the month of August-September according to Gregorian Calendar and the sixth month of Indian (Hindu) Calendar called Bhadrapada or Bhadon.

Reckoning the Day of Thiru Onam
Onam carnival continues for ten days, starting from the day of Atham and culminating on Thiru Onam. These are the most important days for the festivities. The day of Atham is decided by the position of stars. 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.
At this time sun is in the Zodiac sign of Leo (Simha rasi), which happens to be the sun's house as well.

Significance of the Day in Mythology
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.
People believe that it is on the last day, Tiruonam that the spirit of King Maveli visits Kerala, hence the day is marked by feast and festivities. Devotees of Maveli celebrate the golden era witnessed in Maveli's reign during Onam. To welcome their revered ruler, people lay flower mats (Pookalam) in the front courtyard, prepare a grand meal (Onasadya), dance, play and make merry. All this is done to impress upon King Mahabali that his people are prosperous and happy.

A Brief History of Onam
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.

Harvest Time
As it is a 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.