2016
Tuesday, 13 September 2016
In 2016 the nakshatra runs from 10:23 am on 13 Sep to 10:07 am on 14 Sep. It covers the whole madhyahna (11:02 am–1:31 pm) on Tuesday, 13 September 2016, so that is the day of Onam. This year the nakshatra comes 2 times in the solar month; the rule takes the later†.
2017
Sunday, 3 September 2017
In 2017 the nakshatra runs from 9:36 am on 3 Sep to 11:17 am on 4 Sep. It covers the whole madhyahna (11:04 am–1:36 pm) on Sunday, 3 September 2017, but only 13 of its 152 minutes on 4 Sep, so that is the day of Onam.
How the date is fixed
Onam is the day of Shravana nakshatra in the solar month of Simha (while the Sun is in sidereal Simha); the lunar month does not enter. The date is the day whose madhyahna, the middle of the day (the third of five equal parts of the daytime) the nakshatra covers completely. If it covers only part of it on two days, the day on which it covers more of that period is taken†. If the nakshatra comes twice in that solar month, the later one is taken†.
† Practice varies between schools; our faculty is reviewing these rules.
About
Onam is fixed by nakshatra and by the Sun, not by tithi: the main day, Thiruvonam, is the day on which the Moon is in Shravana (Thiruvonam) nakshatra during the Malayalam solar month of Chingam, while the Sun is in sidereal Simha, in August or September. The lunar month does not enter, so the day can fall in Shravana or in Bhadrapada. The day on which the nakshatra holds the middle of the day (madhyahna) is taken, and when Thiruvonam comes twice in Chingam the later one is kept; Kerala's own almanacs occasionally print the next day. The ten days from Atham (Hasta) to Thiruvonam are the festival. It recalls the annual return of King Mahabali to his people.
How it is observed
Flower carpets (pookalam) for ten days, the Onam sadya feast on Thiruvonam, boat races and Kathakali.