Mahavira’s Janma Kalyanak (Mahavir Jayanti) in 1986 fell on:

23 April 1986 (Wednesday) This corresponds to Chaitra Shukla Trayodashi (13th tithi) in the Jain lunar calendar, the traditional date of Bhagwan Mahavir’s birth celebration each year.

You can read a simple explanation about Mahavir Swami’s Janamdin and how the date is decided here