The three-day festival that falls in the month of Karttikai (November-December) is called Appa Karthigai, Vada Karthigai and Thiru Karthigai respectively. The festival coincides with the full moon or pournami day and the constellation of Karthika ( Pleiades). It is said to start from the day of Deepavali. Legend says that Lord Shiva appeared in front of Lord Brahma and Lord Vishnu as they both had a fight over who was supreme. It is believed that each of the competitors went in search of the beginning and the end of the flame that Lord Shiva had changed into, but failed ultimately. Lord Shiva made them realize the ultimate existence of God. This is also known as the festival of Lord Muruga, the son of Lord Shiva and Goddess Paravati. Lord Muruga took the form of six flames that came from the eyes of Lord Shiva.  These flames were further transformed into babies and gathered by Goddess Parvathi.

On this day, kolams or floral patterns are drawn in front of houses and the houses are cleaned and washed.  The doorways are decorated with mango leaf festoons, and lamps, which are commonly known as agal, are lit. Lamps may vary in shapes.  They are called Lakshmi Villaku (shape of a woman with folded hands), Kuthu Vilakku (five-petal flower shapes) and Gajalakshmi Vilakku (shape of an elephant). Devotees observe fast on this day, and it is broken by eating special dishes after the sunset. This is also similar to the Raksha Bandhan festival in North India. Sisters undertake fast for the well-being of their brothers.

A constellation of six clusters that is visible in the night sky resembles a pendant hanging from the ear. The beauty of Karthigai Deepam can be best viewed from the Shiva temples on the hills of Tiruvanna malai. The huge fire on top of the hill is visible for several kilometres.