Why do you suppose Poe sets this story during "the carnival season" in The Cask of Amontillado

I would say that he chose to mix the unexpected setting with the plot of his story in order to only heighten the horrors that happen. The protagonist is plotting someone's murder, and he does that in the middle of a carnival. It is a very odd to depict such a colorful and cheerful scene with such murderous thoughts - however, by doing that, Poe made the story even more impactful.