Die Vecna Die! mentions "A week-long festival, sure to be reenacted on a regular basis." As far as I know, this has never been developed further.
I suggest that this week is celebrated in Mortis, one of the final months of Sigil's Calendar, and that it's called the Festival of Maiming. Named for the Maimed God, this is a celebration of the time Vecna was cast out of the City of Doors, and the multiverse was saved from his meddling. The corpses of criminals are dragged through the streets, their hands cut off and their eyes gouged out in front of cheering celebrants.
As wholesome as this sounds, there are rumors of dark doings during the otherwise innocent Festival of Maiming. In secret places, live victims are maimed, scapegoats for the city's sins. Vecna's cultists have themselves embraced the holiday as their own, celebrating their deity's ascension from demigod to lesser god status, and the most dedicated worshipers maim themselves in imitation of their lord, while traitors to the faith lose the wrong (or both) hands and eyes before being thrown from the Cage. The PCs may be approached to investigate these rumors before the holiday is ruined for everyone. It's like a Christmas special where the protagonists have to rescue Christmas from scrooges, except Santa Claus is a mangled skeleton and all the presents are corpses.
Since you're taking the initiative with this one, Rip, I'll let you decide what makes the final cut.
But some of your suggestions conjured up images of people dressing up in the manner of the Venice Carnival except that the majority of the masks and costumes have the theme of death (e.g. skeletons, grim reapers, eerie clown with ashen faces, etc.) In such an environment, the Vecna cultists could move about relatively unnoticed amidst the revelers
good stuff rip. the last line made me chuckle.
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