Mardi Gras was critical to the story because it provided the perfect cover for the vengeful murder Motressor had planned. The raucous crowds, the drunkeness, overall chaos contributed greatly to his plans of revenge against Fortunato. It is doubtful as to whether or not the plan would have been as successful had it not taken place during Mardi Gras time.
Mardi Gras was crucial to this story because it sets an obvious contrast. While Mardi Gras is a festive and happy time, the reader is a witness to a dark and horrific murder. Mardi Gras also provides the perfect cover for a murder. People are highly intoxicated and carefree. Furthermore, they are donned in Mardi Gras masks making it harder to see the faces of individuals. The happy affair going on above the catacombs and the gruesome murder going on below are nice contrasts to a Poe gothic story.