The choice of the clown’s hat is appropriate because Montresor makes a fool of Fortunato.
Montresor chose to kill Fortunato on the carnival holiday because it is a time when everyone is going to be having a good time and getting drunk. It occurs directly after the Christian Lent, when you give up something. Therefore carnival is when you let loose. Fortunato chose to dress up.
He accosted me with excessive warmth, for he had been drinking much. The man wore motley. He had on a tight-fitting parti-striped dress, and his head was surmounted by the conical cap and bells.
Fortunato is dressed as a jester, or a clown. You may be familiar with this outfit. It was often very colorful, with bright colors such purple and yellow or red in patches (that is the “part-striped” part), and may have pom poms or bells on the end of the cone cap. Also, he is drinking heavily.
The cap and costume is symbolic because Fortunato is dressed as a clown, and acting like a clown (by being silly and drunk) and Montresor is making a fool of him. He is drinking and dressing up because he is participating in a cultural event with some religious significance, since it is related to coming off of Lent. However, Montresor takes advantage of this. He is having a good time, and has no idea that Montresor is mad at him.
THE THOUSAND INJURIES of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge. You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat.
You see, Montresor would never let on that he was angry at Fortunato or threaten him. That would give it all away. Instead, he is going to lure him into the catacombs and murder him. He makes a fool of him by telling him that he has some fancy wine that he is not sure is real. Fortunato, a wine connoisseur who considers himself an expert, cannot resist.
I said to him—“My dear Fortunato, you are luckily met. How remarkably well you are looking to-day. But I have received a pipe of what passes for Amontillado, and I have my doubts.”
Then, once they are in the crypt, Montresor makes sure Fortunato stays by pretending he wants him to go. He tells him that he can just have someone else look at the wine. Fortunato would never allow that. His pride is too big.
“As you are engaged, I am on my way to Luchesi. If any one has a critical turn it is he. He will tell me—”
“Luchesi cannot tell Amontillado from Sherry.”
From then on, the trap is set. Fortunato is not going anywhere. Montresor is such a good judge of character, he knew exactly how to bait the trap, and when to spring it. He knew that Fortunato would be drunk that day and his guard would be down. By the time Fortunato figures out what has happened, he is already bricked into the wall that will be his tomb.