I don’t think the December rewards has been posted yet, unless it was part of a combination topic that I’ve missed:
Edit: Oh I see, there has been an event post on Reddit that didn’t get copied onto here. I’ll do that now.
Nothing completely new,
24th is a Reindeer Mystery Bag:
The ST shards progress as 1x, 2x, 3x, 4x, 5x for a complete 15x = ST chest over the month.
The ST skin shards are 1x and 2x.