Karthigai Deepam is mainly celebrated by Tamil Hindus. It is one of the oldest festivals celebrated by Tamil people. Houses and streets are lit up with rows of oil lamps or Deepam in the evening of the festival day.
Karthigai Deepam and Karthikai Deepam are used interchangeably. Karthigai Deepam takes its name from Nakshatra Karthikai or Krittika. Karthigai Deepam is performed on the day when Krittika Nakshatra is prevailing. Karthigai Deepam is done in honor of Lord Shiva. According to Hindu Mythology on this day Lord Shiva converted himself in an endless flame of light to prove his supremacy to Lord Vishnu and Lord Brahma.
Although Karthigai day is observed every month but the most significant day falls in month of Karthikai (which is same as Solar month Vrischika in other Hindu calendars). DrikPanchang.com lists all Karthikai days in the year including the most significant Karthikai day which is known as Karthikai Deepam.
Hours which are past midnight are suffixed with next day date. In Panchang day starts and ends with sunrise.
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. The fire or Deepam is called Mahadeepam, Hindu devotees visit here and pray to God Shiva.