Used to b a 10 day long festival....Onam week started on Sunday, first Onam falls on Tuesday, followed by Thiru Onam the next day. The third Onam will be observed on Thursday and fourth Onam on Friday.
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Onam is celebrated in the beginning of the month of Chingam, the first month of Malayalam Calendar (Kollavarsham). In 2012, the most important day of Onam (known as Thiru Onam) is on August 29. Rituals commence 10 days before Thiru Onam, on Atham.