What is the symbolism of the Carnival setting?

Poe's "The Cask of Amontillado" is set during the Carnival
/ Mardi Gras setting, using motifs of masks and drink to juxtapose
the atmosphere of merrymaking (above, in the streets) with the revenge plot (below, in
the catacombs).

Traditionally, Carnival / Mardi
Gras is a time of pagan excess before Lent, the 40 self-denying days before
Good Friday (Easter), a time of rebirth and renewal.  However, Poe subverts these
religious themes by exposing Fortunato's excess during Carnival.  He is drunk, sick,
easily duped, unaware of signs, and in denial of his death up until the last jingle of
his conical cap.

The symbolism of the Carnival, in which
party-goers are masked, is ironically undercut when the story's setting shifts to the
catacombs.  There, an unmasked Montressor easily leads Fortunato to his trap.  This
symbolizes that one may be more easily deceived by smiles and desires than by literal
masking.  The fact that a sober friend is more duplicitous than any masked figure
reveals a horrifying irony of human nature.

Montressor (and
Poe) are exposing the Romantic Fortunato's excess in the revelry of the time as it
denies the religious and focus on death.  Poe seems to say that Carnival and the
amontillado are both "red herrings," misleading, not lasting, and false.  Not that
revenge is a better moral alternative, but Montressor and Poe function as morbid
reminders of man's mortality.