Vishnu Deepam is a festival observed on Pournami day in Karthigai masam as per the traditional Tamil panchangam. In 2016, Vishnu Deepam date is December 13.
This day is a major auspicious occasion for Vaishnava sect of devotees. Vishnu Deepam is also observed as Karthigai Deepam. On this day, devotees clean their houses and temples of Lord Vishnu to decorate the places with Deepams (lamps).
It is believed that on this day King Maha Bali was pressed down into the Netherworld (Pathala Lokam) by Lord Vamana. Vishnu Deepam is celebrated with much gusto in Sri Ranganath swamy temple in Sri Rangam, Parthasarathy Temple in Chennai, Ulahalanda Perumal Temple in Kanchipuram, and Varadaraja Temple in Chennai.