I made a post in after December 26 and I did not get the Kofia hat. Is this a bug? Does this mean that you make the first post on the site between 12/26 and 1/1 or does this get awarded when you make your first post in that date range? If the latter, there is a bug.
Description of Kofia hat says
first post between $12/26$ and $1/1$
As mentioned in this MetaSE post
It has to be the first post you did on a site. So if you've already left a question or answer on a site, you can't earn that hat for that site
If you still want to earn your hat you're welcome to make your first post on other SE sites.