Kaarthigai Deepam is a festival of lights, celebrated in the Tamil month of Kaarthigai. It is celebrated on the full moon day of the Kaarthigai month which coincides with Krithikai star. It is also considered as the extension of the Deepavali festival. In some houses, they double the number of lamps every day from the day of Deepavali and this way, they end up with a number of lamps on the day of Kaarthigai Deepam.
It is celebrated in a special manner in Thiruvannamalai. Lord Shiva asks Lord Brahma and Lord Vishnu to find out the exact location of his head and his feet. Since Lord Shiva takes a gigantic form, they are not able to find out anywhere. Then Lord Shiva takes the form of a jyothi (light of fire) on the hill of Thiruvannamali. Therefore, this festival is also known as Annamalai Deepam. Here, a special torch is lighted on the zenith of the hill and it is believed that Lord Shiva's jothi will be visible on this day.
Lord Muruga took the form of six babies in a lake called "Saravana Poigai". On this day, all his six forms were united by Parvathi(his mother) and this way, he had six faces. Special poojas are performed to Lord Muruga.
On this day, people clean the houses. In the evening, they draw kolams (rangoli) in the front of the house and also place some lamps on it. The lamps(Agal) are placed in the pooja and lighted. Then the Deeparathana is done in which the lamps are moved to different places in the house. The lamps glow all over the streets on this day. The lamps are arranged near the doors and windows and also in the balconies. In this way, people of Tamil Nadu celebrate Kaarthigai Deepam for three days.
Source: diwalifestival.org
