I think it depends on the itinerary. When I went on the Western Carib in June, the cruise started on Sunday and the formal nights were Tuesday and Friday. Usually, they try to make formal nights on the days you are at sea, but Friday was Cozumel and many people found it a pain in the butt to have to get back and get dressed up.
The Eastern Carib has three sea days, so I think the formal nights both fall on sea days.
Normally the first formal night is the second night out(Captains Gala Welcome Aboard) and the second formal night is the next-to last night of the cruise - usually on a Sat-Sat cruise it will be Monday and Thursday nights