Currently you can get Twisted Christmas 2010. Both 2011 and 2012 are unlockable all year round so I would only worry about 2010. New achievments and map have also been added. There is a 15 snow globe achievment for the new map as well. There are a couple of exceptions.
In Twisted Christams 2011, A Bloody Christmas Carol
Kill 15 XMas Clots, 5 xmas Stalkers, 5 XMas Crawlers, 1 XMas Siren and 1 XMas Bloat.
That is only unlockable now.
If you've gotten all the achievements I've posted, since I've posted the ones regarding Rachel Clamely, Summer Achievements, Halloween Achievements, and now Christmas, then you've got all the ones that are specifically for a certain time of the year
But It's All Red!
Collect all 16 Z.E.D. Gun pieces.
I'm not even sure when that achievement is unlockable. <_< Supposedly, they are available now in Moonbase. According to some Russian dudes and one American dude commenting on YouTube, they said that you can now get it when the Christmas comes with the new update, and one person stated that they were grateful that they got all the pieces. The comment was 4 days old. Here's the video:
If you missed out on any of the specific ones, then you should be able to get the achievements by looking at their name and realizing what part of the year they would emerge. Though the achievements regarding Transit, I have no clue when they are to become available again.
Happy Achievement Hunting