From Wikipedia, the free encyclopedia Hanuman Jayanti
Hanuman Jayanti is celebrated to commemorate the birth of Hanuman, the Vanara god, widely venerated throughout India especially in North India. It is celebrated on the 15th day of the Shukla Paksha, during the month of Chaitra ( the Chaitra Pournimaa )
Hanuman is an ardent devotee of god Rama, and is worshipped for his unflinching devotion to the God. From early morning, devotees flock Hanuman temples to worship him.
Hanuman Jayanti is an important festival of Hindus. Hanuman is the symbol of strength and energy. Hanuman is said to be able to assume any form at will, wield rocks, move mountains, dart through the air, seize the clouds and rival Garuda in swiftness of flight. He is worshipped in folk tradition as a deity with magical powers and the ability to conquer evil spirits.
The devotees will visit temples and apply tilak of sindhūr to their foreheads from Hanuman's body as there is a common belief that Lord Hanuman loves sindhur. According to the legend Sita was applying sindhūr to her head, Hanuman Ji questioned why was Sita applying sindhur to her forehead. In reply, Sita said that this would ensure a long life for her husband, Lord Rama. Hanuman then smeared his entire body with sindhūr, in an effort to ensure Rama’s immortality.
In Maharashtra, Hanuman Jayanti is celebrated on the full moon day (pūrnima) of the Hindu lunar month of Chaitra. A special feature of Hanuman Jayanti is that according to some religious almanacs (panchāngs) the birthday of Hanuman falls on the fourteenth day (chaturdashi) in the dark fortnight of the month of Ashvin while according to others it falls on the full moon day in the bright fortnight of Chaitra. On this day, in a Hanuman temple spiritual discourses are started at dawn. Hanuman was born at sunrise. At that time the spiritual discourse is stopped and the offering of food (Prasad) is distributed to everyone. 
Spiritual discourses are organised in most of the Hindu temples on this day.
In the states of Tamil Nadu and Kerala, the Hanuman Jayanti is celebrated in the month of Margazhi( normally comes between December 16 to January 14). There it is believed that Hanuman or Anjaneya was born on Moola Nakshatra,on the new moon day (amavasya) in the month of Margazhi.
In Andhra, Hanuman Jayanthi is celebrated on tenth day of bahula paksham in the month of Vaishaka. It is good to parayan Hanuman Chalisa on this auspicious day.