In South India, especially Tamil Nadu Hanuman Jayanthi is celebrated on the Margazhi month of Moola nakshathram. It is said that Lord Hanuman was born during that month.
In Telangana and Andhra Pradesh, Hanuman Jayanthi is celebrated by Diksha of 41 days beginning on Chaitra Purnima and concluding it on the tenth day during Krishna Paksha in the month of Vaishaka..
In Maharashtra, Hanuman Jayanti is celebrated on the full moon day (pūrnima) of the Hindu lunar month of Chaitra.
Why all Hindus won't celebrate Hanuman Jayanti on same day?