Carnival gives Montresor a few advantages for his murderous scheme.
First, carnival guarantees a party atmosphere. This means that Fortunato would be even less guarded than he normally might be. The party atmosphere also ensures that Fortunato will be drunk and drinking more. His inebriation makes him pliable and less resistant to coaxing. Montresor never forces Fortunato to do anything; Montresor coaxes a willing Fortunato.
Second, carnival guarantees that a lot of people will be in Montresor's house. That might appear bad at first, but having a lot of people means that there will be lots of noise. If Montresor has to do something noisy, it won't sound out of place. The large number of people also helps remove suspicion from Montresor. Guaranteed people will notice that Fortunato has been missing for a few days. Many people will have seen him at the party, and many people will assume that he got drunk, wandered away from the party, and something bad happened. There likely wouldn't be much suspicion on Montresor because people would have seen him moving throughout the party all night (before and after Fortunato's burial).