Karthikai Deepam is a Tamil Festival of Lights. The festival is observed in most Tamil homes and every Tamil temple of Tamil Nadu, Sri Lanka, and Kerala. It falls in the month of Kārttikai (mid-November to mid-December) as per Tamil calendar. This occurs on the day when the moon is in conjunction with the constellation Karthigai (Pleiades) and pournami. This constellation appears as a group of six stars in the firmament in the shape of a pendant from the ear. In Kerala, this festival is known as Thrikkarthika. In the rest of India, a related festival called Kartik Purnima is celebrated on a different date.
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courtesy: wedandbeyond