Karthigai Deepam is a Hindu festival which is celebrated mainly by Tamil Hindus. The day of Karthigai Deepam is fixed based on Tamil Solar Calendar. It falls in the month of Karthigai when Karthigai Nakshatra prevails during Ratrimana. This is also the time when Karthigai Nakshatra coincides with Pournami, the full moon day in the month of Karthigai.

Karthigai Deepam festivities at Thiruvannamalai Arunachaleshwara Swamy temple are the most famous one and popularly known as Karthigai Brahmotsavam. Karthigai Deepam festivity at Thiruvannamalai temple spans for 10 days. The celebration begins with Dwajarohanam on the day when Nakshatra Uthiraadam prevails at sunrise. Most of the time, Nakshatra Uthiraadam prevails 10 days before the main Karthigai Deepam day.

Karthigai Deepam should not be confused with Bharani Deepam which is inaugural ritual of Karthigai Deepam. Bharani Deepam is done before sunrise at 4 a.m. . Bharani Deepam day falls one day before or on the same day of Karthigai Deepam. Bharani Deepam is lit inside the temple premises when Bharani Nakshatra prevails. In most years Bharani Deepam is lit at 4 a.m. on the same day of Karthigai Deepam.