It is said that Lord Siva appeared in the form a great flaming cosmic pillar, Lord Brahma and Lord Vishnu requested Lord to take a pleasing form here for the benefit of all beings. On their request, Siva took the form of the Arunachala Hill. Lord declared that "Every year on the day of Karthigai, I will appear on this highest point of the hill in the form of a peace giving beacon"
This festival will be during the months of Nov/Dec ( Varies with Hindu Calendar). As part of this event, a huge bonfire will be built at the summit of Arunachala Hill. This light can be seen from very far distance. Devotees will go for Girivalam, which is a 12 km long trail around the hill.