These things vary from cruise line to cruise line. I'm not familiar with Carnival's policies so I can't give you a definitive answer, but I do know that other lines do hold one formal nights even on cruises of two or three days.
We have been on about 4 of Carnival's cruises to nowhere and there has been a formal night on each one. Alot of the passengers were just as dressed up as on a 7 night cruise. My husband normally wears his tux but on this he is wearing a suit. Cindy