Actually, Christmas decoration disappears a little sooner than its was stated here, it happens on January 10, server save. It includes snowman packages, snowmen from snowman packages (the ones with the name of the char who unpacked it) and snowmen from furniture kits (the ones with no character name on them). However, furniture kits containing snowman does not disappear, but these don't not count for the achievement.
We added this information to TibiaWiki back on January 10, but I just noticed this question now.