how to fix the game...
the game right now is "meh", because after you get the last gun (by which time you have all the upgrades because there's nothing else to spend cash on) the game is out of things for you to do.
But it can be fixed without too much work -
1) change "wave N" to "Dec N"
2) increase the number of foes in days after the 15th so you get the last gun at the end of the 23rd instead of shortly after the beginning of the 24th (since you control the waves and the xp per kill you know precisely when the player will level)
3) make the last gun worth it - a better sound, a more impressive effect, something that makes it funner than the other guns instead of just "a slightly better machine gun"
4) after a full wave (the 24th) with the last gun, though, the game has overstayed its welcome - there's really nothing else to do ... so... END IT. Put a boss (not just a 5x bigger zombie, but something like the evil zombie summoner) Then after you beat the boss say "and so Santa killed all the zombies just in time to deliver presents for Christmas" (that's why I changed waves to days and wanted the 24th to be the last).
The game will still be repetitive and not very original, but at least people who make it to the end will be gratified a little and give it more stars because the game didn't presume to stick around longer than it had anything to offer.