Purcell had the right idea except he should be using elseif instead of else and if's and he should be using $1- not $1- and furthermore you should strip $1 so that if the person triggering is using a color talker it will still work.
I will use /tokenize 32 $strip($1-) to make $1 be stripped thru the whole script so that I dont have use a $strip call in ever if statement.
I also noticed a typo in the christmas greeting, it was missing a ctrl+b and ctrl+k.
Code edited:

on *:text:*:#:{ 
  tokenize 32 $strip($1-)
  if ($1- == !xmas) {
    msg $chan 4M3E4R3R4Y 
    msg $chan 3C4H3R4I3S4T3M4A3S4!3! 
  }
  elseif ($1- == !easter) {
    msg $chan 5H6A13P9P4Y __()()
    msg $chan ________(o.o)
    msg $chan 3E4A12S10T9E7R13! (( . ))
  }
  elseif ($1- == !halleween) { msg $chan 7,1 HAPPY HALLOWEEN! Trick or Treat!  }

  elseif ($1- == !birthday) { msg $chan 12H3A4P5P7Y 13B6I9R8T14H2D11A10Y12 $2- 4! 9! 7! }

  elseif ($1- == !newyear) { msg $chan 4,1*\o/* 13*\o/* 8HAPPY NEW YEAR! 7*\o/* 12*\o/* }
}

Good luck.