In Jain tradition, both words point to the same thing here:

• “Janma Kalyanak” = the birth kalyanak of Bhagwan Mahavir
• “Mahavir Jayanti” = the festival celebrating that same Janma Kalyanak

So tomorrow (31 March 2026) is Mahavir Swami Janma Kalyanak, which people commonly call Mahavir Jayanti.

There is no difference of two separate festivals – it is one occasion with two names:

• more traditional/scriptural term: Mahavir Janma Kalyanak
• popular/common term: Mahavir Jayanti

You can read a simple explanation of Mahavir Janma Kalyanak here