Wowhead's guide states that, while they're here today, these gifts will be gone tomorrow, so grab your last chance to get the Winter's Helper pets, the Mechanical Greench, the Snowman Kit or Jingling Bell. There's also some eggnog, cookies, wands, and roast dinners to be had, amongst other surprises. And, how could I forget, the achievements that go with them. Of course, any of these items that are limited to Winter Veil only have a day left in them, so make haste, don't tarry, open a present today! You can only enjoy the Mechanical Greench for so long.
So if you're missing just one part of the Winter Veil achievement set for your Long Strange Trip, today's the day. It's back to just being plain old January tomorrow. No wonder that Overseer looks so glum.