On the blog we are told "that on January 2 all hats will be taken back to storage."
Does that mean the Winter Bash includes the second or is the first the last day?
Times help clear up ambiguity: does WB end on Jan 2 0000 UCT or Jan 3 0000 UCT?
From the FAQ
What is going on? Why do I see hats?
From December 12th through January 1st, you'll be able to earn hats all over the sites!
That means that Winter Bash 2018 ends on 2019-01-02 00:00 UTC.