Karthikai Deepam is a festival of lights celebrated by Tamil Hindus on the full moon day of Karthikai month.
This occurs on the day when the moon is in conjunction with the constellation Karthigai (Pleiades) and purnima.
The lighted lamp is considered an auspicious symbol. It is believed to ward off evil forces and usher in prosperity and joy.
Karthigai festival in Tiruvannamalai hills is very famous. On Karthigai day, a huge fire lamp is lit up on the hill, visible for several kilometers around the hill.
Web : http://en.wikipedia.org/wiki/Karthikai_Deepam