The reference in Edgar Allan Poe's short story The Cask of Amontillado to "the carnival season" is not intended to establish as its setting an actual "carnival" as the word is employed in America. Rather, "carnival season" within the context of Poe's story refers to the week-long carnival-like atmosphere that preceded the period of Lent in Italy. Its closest American approximation would be the annual Mardi Gras celebration in New Orleans that caps the end of Carnival season in that Louisiana city. The streets are filled with people in costumes, and there is a joyful atmosphere throughout the community. That is to what Poe's narrator, Montresor, is referring when he mentions his fortuitous encounter with Fortunato.
That said, Montresor is pleased to happen upon Fortunato during the carnival season for two reasons. First, Montresor has been obsessed with avenging a series of insults he believes he has received at the hands of Fortunato. The story's opening sentence is quite explicit on this matter:
"THE thousand injuries of Fortunato I had borne as I best could; but when he ventured upon insult, I vowed revenge."
The second, and related reason why Montresor is pleased by this encounter is that he knows his nemesis is inebriated, Fortunato having partaken of a great deal of alcoholic beverage as part of the annual festivities ("He accosted me with excessive warmth, for he had been drinking much"). Montresor knows that Fortunato, a respected and feared man, will be easier to manipulate while under the influence of alcohol, especially when the former wisely appeals to the vanity of the latter. In short, Montresor is pleased by this encounter because it plays into his hands. He has planned the execution of Fortunato, and the carnival atmosphere will make his task much easier.