Pantheon - Every first Sunday of the month the entrance is free.

There are no tickets or reservations available for State museums and cultural sites on Free Sundays…you just get in the usually very long lines and wait your turn. The only exception I can think of is the Borghese Gallery where you still need to prebook a time slot.

The most expensive way to visit any major site is on 'free Sunday'

I guess we'll just have to get lucky. Thanks.