Garbh (Chyavan) Kalyanak of Bhagwan Mahavir is celebrated on:

Chaitra Sud 13 (13th day of the bright half of the month of Chaitra, according to the Jain lunar calendar)

This same tithi (Chaitra Sud 13) is also observed for His Janma Kalyanak. Both Shwetambar and Digambar traditions accept Chaitra Sud 13 as the tithi for Mahavir Prabhu’s Garbh/Chyavan Kalyanak and Janma Kalyanak.

You can read a simple explanation of Mahavir Bhagwan’s Kalyanaks here