In my world it's a holiday when the barrier between the realm of the living and dead are blurred, so ghosts are able to interact with the living. It has often resulted in quite a manner of mischief, so the holiday started with people either dressing up as ghosts to avoid being a target, or placating the spirits with offerings of candy and sweets. Of course over the years costumes grew more varied and extravagant, embracing all a manner of beast and monster under the guise of being the "ghost of" such beings.
Not every world seems to share the backstory, but I have noticed that costumes, sweets, and lanterns of some sort are a common theme across other worlds.
And what better way would there be to show off our combined skills than a gathering centered around costumes? The nature of the costumes allows for a fair amount of artistic freedom, and it gives people the opportunity to throw away their inhibitions and dress in ways they normally wouldn't.
Vil goes all-out for the holiday it would be perfect
Not every world seems to share the backstory, but I have noticed that costumes, sweets, and lanterns of some sort are a common theme across other worlds.
And what better way would there be to show off our combined skills than a gathering centered around costumes? The nature of the costumes allows for a fair amount of artistic freedom, and it gives people the opportunity to throw away their inhibitions and dress in ways they normally wouldn't.