Pongal in Tamil means “boiling over or spill over”. The main ceremony of the festival is letting rice boil over with milk in a clay pot symbolising future prosperity for the family. The second day is the main day, where the ’sarkkarai ponggal’ dish is boiled with fresh milk in new pots during prayers. The moment the rice boils over and bubbles out of the clay pot, they exclaim "Ponggalo Pongal". It is considered a good sign to watch mixture boil over, since it means good luck and prosperity. The festival usually takes place in mid January.
More details at: http://www.pongalfestival.org/pongal-festival.html