In Tamil Nadu Hanumath Jayanthi is observed during Margashirsha Amavasya. Most of the time, Margashirsha Amavasya coincides with Moolam Nakshatram. It is believed that God Hanumath was born during Margashirsha Amavasya when Moolam Nakshatram was prevailing. In years when Moolam Nakshatra doesn't coincide with Amavasya, Amavasya day is preferred to decide Hanumath Jayanthi day. In Gregorian calendar Tamil Hanumath Jayanthi falls in January or December.
In North Indian states, Hanumath Jayanthi is known as Hanuman Jayanti and it is observed on full moon day during Chaitra month. Hanuman, who is also known as Vanara God, was born on this day and Hanumath Jayanthi is celebrated to commemorate the birth of Hanuman.
Devotees observe Hanumath Jayanthi during different time of the year according to their regional beliefs and the type of calendar being followed. Hanumath Jayanthi during Chaitra Purnima is the most popular one in North Indian states.
In Andhra, Hanuman Jayanthi is celebrated for 41-days which starts on Chaitra Purnima and ends on the tenth day during Krishna Paksha in Vaishakha month. In Andhra Pradesh devotees begin 41-days Deeksha on Chaitra Purnima and conclude it on Hanuman Jayanthi day.