Hayagriva Jayanti celebrates the birthday of Hayagriva, an incarnation of Lord Vishnu. In the Hayagriva incarnation, Lord Vishnu appears with the head of a horse rescues the Vedas that were stolen by the demons. In
South India, the day is observed as Upakarma and Avani Avittam by Brahmins. In 2016, the date of Hayagriva Jayanthi is August 18.
Hayagriva is one of the lesser known incarnations of Lord Vishnu. Hayagriva, the horse-headed Vishnu, is considered to be the guardian of Vedas. Hayagriva Avatar is mainly mentioned in the Shantiparva of Mahabharat and the Puranas.
In the main legend associated with Hayagriva, Devas led by Indra are the villains. It is said the powers of Vishnu made Indra jealous and they decided to kill him. Once the Devas found Lord Vishnu resting with his chin on his bow. The Devas turned into termites and chewed one end of the tight bowstring until it snapped. The bowstring snapped with such force that it beheaded Vishnu.
Without Vishnu, the Devas were helpless before Asuras (Demons). Asuras used this opportunity and stole the Vedas and overpowered Devas. Indra and gods were ashamed of their unwisely act and decided to resurrect Vishnu but they could only find his body; the head was missing. It is said that the Devas placed the head of a horse and resurrected Vishnu. Hayagriva, defeated the demons, and brought back the Vedas.
There is another version of the same story which tries to cover up the unholy act performed by Devas as a mistake. You can read that legend here.