The full moon day in the Tamil month of Karthigai is observed as Vishnu Deepam by Lord Vishnu devotees (Vaishnavites). The day is popularly known as Karthigai Deepam – when all houses and temples will be decorated with oil lamps after sunset. Vishnu Deepam is celebrated by lighting bonfire known as Sokkappanai in Vishnu shrines. The reason for the festival is based on Lord Vamana (Lord Thrivikrama) incarnation of Lord Vishnu. In 2013, the date of Vishnu Deepam is November 17.
Lord Vamana sending King Bali to the underworld is the basis of Vishnu Deepam. 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. The bonfire lit on Vishnu Deepam remembers this event.
Vishnu Deepam is also observed as Yanai Pandigai by some communities in