Let me clear up a bit of the question. The beginning of the story is not happy and fun. Montresor spends the first three paragraphs telling his reader about how awful a person Fortunato is. Montresor emphatically declares that Fortunato must be punished.
I must not only punish, but punish with impunity.
Montresor follows that up by alerting his reader to Fortunato's weakness that he plans to exploit.
He had a weak point—this Fortunato— . . . He prided himself on his connoisseurship in wine.
The carnival season is introduced in the fourth paragraph, and it is a drastic shift in tone from the previous three paragraphs. Montresor goes from maniacally plotting his revenge to the face of pure bliss as he greets Fortunato.
I was so pleased to see him, that I thought I should never have done wringing his hand.
I said to him: “My dear Fortunato, you are luckily met. How remarkably well you are looking to-day!
Next, Montresor offers up to share a ridiculously expensive bottle of wine with Fortunato. It appears that they are best of friends again. The fact that it is carnival season tops everything off, because the reader begins to think that maybe Montresor has changed his mind. Or at least he isn't willing to spoil the fun of carnival by further focusing on revenge.
Then the reader finds out just how dastardly Montresor is being. He planned his revenge to be during carnival, so that when Fortunato doesn't show up the next day, everybody will assume he's passed out drunk somewhere. Carnival also gave Montresor an easy way to get rid of his household servants.
There were no attendants at home; they had absconded to make merry in honor of the time. I had told them that I should not return until the morning, and had given them explicit orders not to stir from the house.
And lastly, Fortunato is already so drunk that he doesn't suspect anything other than more drinking and more merriment.
Montresor essentially found a way to bury away all that fun of carnival. On the other hand, it was a carnival that he'll never forget.