Guild Wars 2

Will the material I’ve collected from the wintersday gifts become useless/worthless after wintersday ends?

pretty much. Until the next wintersday. Ugly sweaters, socks, hats have been a thing in all wintersdays so far. So just plop them in your bank

Snowflakes can be used for crafting whenever though so they’re not useless.

the npc vendors will eventually go away, so I would definately use them before the event ends.

Since the Ugly Garments will be obtainable through the Generosity’s Reward, it is possible vendor(s) may permanently exchange these items.

Guess we will find out later in January.

Good luck.