Yeah. You want it to be 0 so it will keep checking at 1 minute past midnight every day. When it is the correct day, it will turn the timer off.

Just as a note to build off what I posted... You may want to start that timer in an on START event so that it will not stop running when you close/open mIRC.

Code:
on *:start: {
  .timerchristmas 0 0:01 checkdate
}
alias checkdate {
  if ($date == 24/12/2006) {
    topic #yourchannel Merry Christmas!
    .timerchristmas off
  }
}


And, yes, you put this into a new remote file... Go into remotes, select File > New, and paste.

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