Merry Golfmas, Everyone!

Pyke, 21 bytes

.dↄґ6C65h*325q/Al

Try it here!

     C65h*        -   multiply the day by the (current month + 1)
          325q    -  ^ == 325
    6         /   - 6 / ^
.dↄґ              -  load "merry christmas"
               Al - ^.capwords()

Or 18 bytes noncompetitive.

.dↄґ6Cs6Y1q/Al

Exactly the same except for this section:

Cs6Y1q 
C      -    get_time()
 s6    -   add 1 week
   Y   -  get day of the year
    1q - ^ == 1

Try it here!

JavaScript, 55 53 46 bytes

^{Note: this has only been tested in Google Chrome, program may behave differently from browser to browser (or device to device)}

2 bytes saved thanks to @ConorO'Brien

7 bytes saved thanks to @ETHProductions

alert(/c 25/.test(Date())?"Merry Christmas":a)

Exits with Uncaught ReferenceError: a is not defined if the date is not Dec 25. I'm not really sure if this counts as crashing

alert(/c 25/.test(Date())?"Merry Christmas":a)

PHP, 39 38 bytes, not competing (doesn´t crash)

<?=date(md)-1225?"":"Merry Christmas";

or

<?date(md)-1225?die:0?>Merry Christmas

or

<?=["Merry Christmas"][date(md)-1225];