In the fourth paragraph of Edgar Allan Poe's short story "The Cask of Amontillado," the narrator, Montresor, describes his reaction to seeing Fortunato on the streets during carnival season:
I was so pleased to see him that I thought I should never have done wringing his hand.
In other words, he was so happy to see Fortunato because he knew it was such a perfect moment to exact his revenge that he couldn't stop shaking Fortunato's hand. While he is not genuinely happy to see Forunato, he is happy that such a perfect moment for revenge has arisen. Continuing to shake Fortunato's hand added the extra benefit of putting Fortunato at ease, making him believe that they truly are friends when the exact opposite is true.
The phrase "carnival season" most likely refers to the Carnival celebrated all over Italy that starts during Epiphany, January 6th, and lasts until the start of Lent. It is a time of revelry and unrestrained celebration that dates back to ancient Roman times in which the Roman god Saturnalia was celebrated to ring in the new year. The Roman Catholic Church turned the pagan holiday into a time of celebration before total abstinence from all worldly pleasures must be upheld during Lent, six weeks of prayerful penance before Easter Sunday. Since the story is set during Italy's Carnival, Fortunato appears dressed in motley, which means he is wearing the multi-colored costume worn by court jesters and clowns.
Poe's choice to set the story during Carnival has significance for three reasons. First, the time of celebration gives Montresor the perfect excuse to feign delight at seeing Fortunato. Second, since it is during Carnival, by the time anyone realizes Fortunato has gone missing, anyone looking for him will attribute his disappearance to something having happened to him due to his own drunken revelry. And, third, since Montresor is murdering Fortunato through abandonment and starvation just before Lent, we know Lent will absolutely not be a time of prayerful penance for Montresor.