Thats strange because i definately bought the game after the january-patch and was able to play that mission. Was this intentional?
Yes it was. The christmas mission was only available for a limited time. I guess the last version on GOG that had the "Holiday Event" included was v.188.8.131.52 and it was removed again in v.184.108.40.206. Now with v.220.127.116.11 it's back again. Not sure for how long though. It could possibly be removed again with the next update, so better save a backup of the 18.104.22.168 classic installers if you want to be able to play the "Wang in the Box" mission again.