Vishnu Deepam is observed on the full moon day or a day earlier in Karthigai Masam as per traditional Tamil Calendar. Vishnu Deepam 2015 date is November 25. It is observed by lighting bonfire known as Sokkappanai and is based on the legend of Thrivikrama (Vamana Avatar) of Lord Vishnu.
The legend associated with Vishnu Deepam is an interpretation of Lord Vamana asking three steps of land from King Bali. Here instead of land it is based on the movement of sun and earth. It is said that the two steps of Lord Vamana are the Uttarayan and vernal equinox. And the third step which sends
Bali to the underworld is the autumn equinox and the bonfire lit on Vishnu Deepam remembers this event.
Karthigai Deepam is also observed on the day and this is mainly dedicated to Lord Shiva and Lord Muruga.