Datta Jayanti (Dattatreya Jayanti) is the first appearance day of Lord Dattatreya, the avatara of Trimurthi (Brahma, Vishnu & Maheshwara). In 2013, Datta Jayanti date is 16/17 December.
Datta Jayanti is observed on Margashirsha Purnima, Full Moon day in Margashirsha Month of Hindu calendar. In some places it is also celebrated on a day prior to Margashirsha Purnima, Margashirsha Shukla Chaturdasi.
Datta Jayanti is celebrated with utmost gusto in all Temples dedicated to Sri Dattatreya Avatara and Avadhutas. Akkalkot in Solapur district of Maharashtra, Gangapur in Gulbarga district of Karnataka, Pithapuram Sripada Srivallabha Temple in Andhra Pradesh, and Shirdi Saibaba Temple in Maharashtra.
Read this post to know about Datta Jayanti puja procedure and the Vrat Vidhi for Dattatreya Jayanti.