Hanuman Jayanti marks the appearance of Hanuman, the vanara whose devotion to Rama and Sita is the model held up in the Ramayana and the Hanuman Chalisa. Devotees read the Chalisa and the Sundara Kanda, apply vermilion and oil to his images, offer boondi and jaggery, and many keep a fast or visit a Hanuman temple before dawn.
In the north the birthday is kept on the full-moon day (Purnima) of the month of Chaitra. Several other regions observe it on different days: parts of the south, particularly Tamil Nadu, keep it in the month of Margashirsha, Karnataka around the same period, and Andhra and Telangana begin a forty-one-day observance from Chaitra Purnima that ends in Vaishakha. Maharashtra follows the Chaitra Purnima date and times the celebration to sunrise, the hour of his birth.
The variation comes from different traditional texts and regional calendars rather than a single agreed date.