I'd be interested in participating in an event, but I don't think that I will have the time to actually run something.
I'd be tempted to suggest changing the clocks and rolling the characters early so that people don't have to play precisely on Christmas or New Year's if you want to do anything major.
First thing that popped to mind... maybe a 12 days of Christmas type challenge? Each "day" (player turn), you need to find something specific using a new character. It could get grindy if you aren't careful in how you plan it, but something like:
On the first day of Christmas the RNG gave to me:
A cute dog brought home safely.
On the 2nd day of Christmas the RNG gave to me:
Two broken weapons
And a cute dog brought home safely.
On the 3rd day of Christmas the RNG gave to me:
Three dead goodwives
Two broken weapons
And a cute dog brought home safely
etc.
Hoping to win with every class, doomed. Archer, Barbarian, Bard, Beastfighter, Druid, Elementalist, Farmer, Fighter, Monk, and ULE Priest down.