It seems that 11 bit studios just sell the game and doesn't care about customers' support after they get the money
Fine, I found the answer by myself. For those who care about the same problem, just move here:
http://forums.steampowered.com/forums/s ... ?t=2517809The answer is negative. You can't finish this achievement since it's time limit and has been blocked by the developers.
I have never seen a game on steam have this blocked achievement you can't get it only because you miss some "winter sales".
Suggestion:
1.add a bonus level to get Santa's gifts.
or
2.if the date of the computer meets certain condition, it will trigger the air support to drop Santa's gifts. Just like other games deal with coal achievement. (EDGE, Cave Story, DEFCON, Toki Tori etc.)
One can change their PC date or just wait for next Christmas.