This is in the spirit of the What is a Word/Phrase™ series started by JLee with a special brand of Phrase™ and Word™ puzzles.
If a word conforms to a special rule, I call it a X-Mas Word™.
Use the following examples to find the rule.
Here is a CSV version:
X-MAS WORDS™, NOT X-MAS WORDS™ WHISKEY*, RUM CHRISTMAS, HANUKKAH HAPPY*, SAD JOY, FUN CHOCOLATE*, COCOA YOGHURT*, MILK IDIOT*, DUMB GLAMOUR, FLAIR BABY*, CHILD EXOTIC*, PLAIN WINTER*, SPRING SNOWBOARDING*, SWIMMING LIGHT, DARK FOOD*, DRINK JINGLE*, MUSIC SANTA CLAUS, MICKEY MOUSE TOY, BALL HEAT*, CHILL SNOW, SEA FAILURE, SUCCESS DROP, DRIP DEER*, MOOSE
*Extra X-Mas Word™
The puzzle relies on the series' inbuilt assumption, that each word can be tested for whether it is a X-Mas Word™ without relying on the other words.
These are not the only examples of X-Mas Words™, many more exist and can be found.