Like a "Steam Christmas Sale" or something to that effect? That'd be pretty cool.
I think it'd be great to have it done in a way that benefits the site as well as it's users.
Maybe done through submissions to GB? Like say if a user submits 4 items (whatever category) within a certain period (December?) they get a "Christmas/Holidayunlock bonus."
The unlock bonus could be a reduced GB point pricing on unlocks, for a limited time on a limited amount of unlocks.
This would encourage further GB submissions and member activity in that period.
If planned strategically such an event could be used to boost months of statistically lower user activity throughout the year. Instead of Holiday theme, could be seasonal like Steam does it (GB "Spring Unlock Event").