Guild Wars 2

Will the wintersday trophy items (ex. ugly wool socks) be useable for next winter event?

Probably.

I don’t think anyone knows for sure (Anet may not even have decided) but I think based on GW1 festivals and my own experience with other games it’s likely that at least some aspects will be repeated. Unless they scrapped everything and re-did it all it would makes sense for the trophies to be one of the things which are re-used.