According to a post on the unofficial Guild Wars website, the Guild Wars' Wintersday Event will have to be re-run.

The original Wintersday event, which took place over the holidays, was apparently plagued with a variety of bug-related issues that almost rendered the holiday event unplayable. In particular, while players complained heavily about the lag of the burdened Guild Wars servers, a large amount of the player base was simply unable to get their Wintersday hats.

Upon discovery of this bug, however, Regina Buenaobra, the ArenaNet Community Manager, did some fantastic footwork communicating between the players and the team to fix the problem as quickly as they could. Now, however, as a way of making it up to the players who just didn't get their hats, Regina has promised that the team will be re-running their Wintersday event, so everyone will be happy!