Tamil calendar

The Tamil calendar is a solar calendar. A month is the time the Sun spends in one sidereal sign: Chithirai while it is in Mesha (Aries), Vaikasi in Rishabha, and so on to Panguni in Meena (Pisces). So the months have 29 to 32 days and begin around the middle of the Gregorian month, and the same Tamil date falls on almost the same Gregorian date every year.

Which day is the first of a month depends on when the Sun enters the sign — the sankranti. If it enters between sunrise and sunset, the month begins that day; if it enters after sunset, the month begins the next day. An entry after midnight but before sunrise belongs to the night before, so the month begins on that calendar date. This is the rule Tamil almanacs and Drik Panchang use for Tamil Nadu, and our calendar applies it to Chennai's sunrise and sunset.

The year begins on Chithirai 1, Puthandu, and is named from a cycle of sixty: Prabhava, Vibhava, Sukla and so on to Akshaya, after which the cycle starts again. Tamil Nadu's government also counts the Thiruvalluvar year, which turns on Thai 1, the day of Pongal.

Festivals of the lunar calendar — Vinayagar Chathurthi, Navarathiri, Deepavali, Skanda Sashti — keep their tithi; festivals of the Tamil calendar proper are tied to the solar month: a nakshatra in it (Thai Poosam, Masi Magam, Panguni Uthiram, Karthigai Deepam, Arudra Darshan), its new or full moon (Thai Amavasai, Aadi Amavasai, Chitra Pournami), or a day number (Aadi Perukku is Aadi 18).

We follow the Thirukanitham (computed) reckoning. Vakyam almanacs, which some temples follow, use older tables for the Sun and the Moon and can put a month start or a festival one day apart; when your temple's calendar says otherwise, follow your temple.