Karthikai deepam is an ancient festival of lights celebrated by Tamil Hindus on the full moon day of Karthikai Month (November/December).  This occurs on the day when the moon is in conjunction with the constellation Karthigai (Pleiades) and Purnima.  It is the same as Kartik Purnima;  since tamils follow the Hindu Solar calendar with correction for precession of the equinoxes, the Tamil date matches the actual constellation.