Hanuman Jayanti is the birthday of Sri Hanuman and it is observed on the Moolam Nakshatra day in Margazhi month in Tamil Nadu. The January 4, 2011 is only observed in Tamil Culture. In North India and western parts of India, Hanuman Jayanti is observed on the Chaitra Purnima day. In 2011, this is on April 18.
Read more about Hanuman Jayanti in Margazhi month.