yeah, it's jsut the katakana version of merry christmas. Well japanese dont have christmas so, that phrase is foreign to them. They use Katakana to write foreign words and they change them a bit so it's easier for japanese people to pronounce
Me-rii Ku-ri-su-ma-su . If you say that fast it sounds like Merry christmas, sort of
But that's almost correct.
メーリ クリスマス seems more correct. That's Mee-ri Ku-ri-su-ma-su. That's because we want to extend the sound of Me instead of the sound of ri, in the first word
Anywayz, have fun