Thiruvonam

Kerala's harvest festival, marking the annual return of king Mahabali. Thiruvonam is the tenth and principal day of the ten-day observance.

Onam is dated in the Malayalam solar calendar, not the lunar one: it is the day the nakshatra Thiruvonam — Shravana — runs while the Sun is in Simha, the month Malayalis call Chingam. Chingam runs about 31 days and a nakshatra returns every 27, so occasionally two Thiruvonams fall inside the month; the later one is kept, because the ten days of Onam count backwards from it to Atham and that run has to fit inside Chingam too.

• Verified against observed Onam dates 2020-2025; 2024 is the only year in that span with two Thiruvonams in Chingam, and Kerala kept the later one (15 September).
• The nakshatra is taken as the one running at sunrise.