Carnival season is an "upside down" time of partying and revels. Montresor calls it a season of "extreme madness," one ideal for enacting his mad plan of walling up his hated rival, Fortunato, in a catacomb.
In this first place, Fortunato is already drunk when Montresor runs into him. Fortunato has been "drinking much," Montresor says. Therefore, Fortunato's judgment was already impaired, and he was unlikely to question what was happening until it was too late. Further, the season gives Montresor the excuse to tell his servants he will be gone all night and to forbid them to leave the house, knowing that as soon as he is gone they too will all head out to enjoy the carnival. This leaves the house empty so that Montresor can enact his plan without witnesses or interference.
Finally, Montresor is going for effect and the carnival is a perfect backdrop for his evil plan. There is something delicious in Fortunato going to his doom in a cap jingling with bells and a parti-striped outfit. Also, during this period of madness, nobody is likely to notice Fortunato's disappearance until it is far too late--and even then, might assume he died in an accident.