When is Thiru Onam

Onam Festival is celebrated in the beginning of the first month of Malayalam Calendar Kolla Varsham called Chingam. This month corresponds to August September in Gregorian Calendar and Bhadrapada or Bhadon in Indian Hindu Calendar.Onam carnival continues for ten days, starting from the day of Atham and culminating on Thiru Onam. Atham and Thiru Onam are the most important days for Onam festivities. The day of Atham is decided by the position of stars. Onam 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. Thiru Onam corresponds to the 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 house as well.The dayof Onam 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 sent 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 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.