How the date is fixed
Thai Pongal is the day the Sun enters sidereal Makara. It is a solar festival, so its Gregorian date stays nearly the same every year. If the Sun enters the sign after sunset, the solar month, and Thai Pongal, begin the next day (the Tamil reckoning)†.
About
Thai Pongal is the Tamil harvest festival, kept on the first day of the solar month Thai, which begins when the Sun enters sidereal Makara (Capricorn), on 14 or 15 January. The Tamil calendar starts the month on the day of the sankranti if the Sun enters the sign before sunset, and on the next day if it enters after sunset, so Pongal can fall a day after the Makar Sankranti that other calendars print. It is the second of four days: Bhogi the day before, then Thai Pongal, Mattu Pongal for the cattle, and Kaanum Pongal. The name comes from the dish of new rice and milk that is boiled until it spills over.
How it is observed
New rice and milk are boiled in a new pot until they overflow and offered to the Sun in the open; kolam designs are drawn at the doorway.