Full moon days in Hindu calendar are known as Purnima or Pournami.
South Indian people called it as Pournami, whereas North Indians called it as Purnima.
Apart from festivals, Pournami is a very important and auspicious day in Hindu calendar.
Pournami that falls in the month of Mrigashira or Margashirsha or Margazhi is called as Margashirsha Purnima or Margashira Pournami.
Dattatreya Jayanthi is observed on this day, special rituals will be held in temples dedicated to Lord Dattatreya (incarnation of Trimurtis).
In some regions Margashirsha Purnami is called as Battissi Purnima, Korala Purnima, Margashirsha Poonam, Naraka Purnima or Udyatithi Purnima.