Not much for a reply from Carnival but this is from thier own website.
How many formal nights are there?
For cruises with one formal night, the formal night is scheduled for the second or third evening of the cruise, with schedules permitting. For cruises with two formal nights, the first formal night is scheduled for the second or third evening of the cruise; the second formal night is scheduled for the second or third evening prior to the end of the cruise, with schedules permitting.
1-day cruise: no formal nights
2-day cruise: one formal night
3-day cruise: one formal night
4-day cruise: one formal night
5-day cruise: one formal night
6-day cruise: two formal nights
7-day cruise: two formal nights
I know it's been four years but what happen to Causal/Semi Formal/Formal nights?
On my 7 day Carnival cruise, there were 2 formal nights. The most formal being the second night of the cruise which also includes the captains cocktail party. The other was the 6th night but was much less formal. I would say even semi-formal may be an overstatement. I just wore a nice dress (not a cocktail dress), hubby wore Dockers and a casual button up shirt (no tie, not that type of shirt). Actually, we weren't dressed much more formally for the major formal night either. Hubby skipped the dining room for room service and my teen daughters and I wore knee length "little black dresses" and did not feel out of place.