When is the festival?

Answer 1

Onam is celebrated in the beginning of the month of Chingam, the first month of Malayalam Calendar (Kollavarsham). The most important day of Onam (known as Thiru Onam) is the second day of four days of Onam. Festivities actually commence on Atham, around 10 days earlier, with the preparation of pookalam (floral arrangements) in front of homes.

Translating dates to the US Calendar, Onam usually happens sometime in late August through Mid-September, depending on the year. In 2015, Thiru Onam was August 28. In 2016 and 2017 it is September 13th and September 4th, respectively.

The festival commemorates the appearance of Vamana avatar of Vishnu and the subsequent homecoming of the legendary Emperor Mahabali. Onam celebrates the Asura King Mahabali's annual visit from Patala (the underworld). On Thiruvonam day, Asura King Mahabali is believed to visit every Malayali home and meet his people.

Sources: about.com and drikpanchang.com