cupcake, here is the simple countdown snippet modified as per your question:

alias countdown {
  noop $$?="How many minutes apart would you like me to announce the countdown?"
  var %c amsg $duration($calc($ctime(Dec 24 2010 11:59) - $ctime))
  $+(.timercd,$cid,$network) 0 $calc($! * 60) %c left until Christmas.
} 

Put this in your mirc remote and enter the number in minutes. For instance, 1 equals one minute and so on and so forth. Once the minute is set, the snippet will announce the countdown in the minute you've specified.

The command is:

/countdown

This is a rough mock-up of snippet. There is no check for the Christmas when it's due. So on Christmas, you may need to remove this snippet and restart your mirc.