Once Brahma and vishnu due to the ego started arguing with each other " who is the greatest". Both claimed to be the greatest, instead of doing their duty. At that time the god shiva appeared in the form of fire and told them whoever finds the top or bottom of the fire is the greatest. Brahma took the form of Annam (a bird) and tried to find the top of the frle pillar. vishnu took the form of Varaha( giant pig) and tried to see the bottom of the pillar. As the god has no limit in his size or power, both of them could not see either top or bottom of the fire. When their ego went down and they prayed the god, He appeared before them.
Indeed, the very names `Tiruvannamalai' and `Arunachala' translate as `holy fire hill.' The Shivalinga in the temple here is the agni linga. Every year thousands of devotees from chennai and elsewhere flock to Tiruvannamalai to see the spectacular Karthigai Deepam there.This vradha indicates the non measurable infiniteness of the god and shows that the god is the light that will drive away our ignorance.
First day of this festival is called as "Appakarthigai" and the second day is called as "vadai karthigai"