Hanuman Jayanti is observed as the birth anniversary of Lord Hanuman. The day is very significant for the devotees of Lord Hanuman. Lord Hanuman is the son of Mata Anjana and Kesari. He is also described as the son of Vayu Deva i.e. the wind god.
There is no consensus on the exact day of Hanuman Jayanti in Hindu calendar. Hence, the day to observe Hanuman Jayanti varies as per regional beliefs. However, Chaitra Purnima is the widely accepted day in most North Indian states to observe the birth anniversary of Lord Hanuman.
Lord Hanuman is an ardent devotee of Lord Rama and He is the central figure of Hindu epic Ramayana. Hence the origin of the birth anniversary of Lord Hanuman goes back to Era of Lord Rama. Hence, it is difficult to find the exact time of Hanuman Jayanti as it is getting observed since ages (as per some unconfirmed records Lord Hanuman was born approximately 1 crore and 85 lakhs years before).
It is believed that Lord Hanuman was born during Chaitra Purnima just after sunrise on weekday Mangalwar. He was born during Chitra Nakshatra and Mesha Lagna.
Lord Hanuman is the main deity who is worshipped on the day of Hanuman Jayanti.
As per Amanta as well as Purnimanta Hindu Calendar -
Purnima (the full moon day) of Chaitra (1st month)