What is a Christmas Word™? (also: Connect the Clues!)

Question

_{In the spirit of the What is a Word™/Phrase™ series started by JLee, a special brand of Phrase™ and Word™ puzzles.}

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'Tis the season ....

If a word conforms to a special rule, I call it a Christmas Word™.
Use the following examples below to find the rule.

Christmas Words™	Not Christmas Words™
DO	LO
BE	LET
GOD	LAY
HAS	ALL
JOY	OLD
GORY	BLOODY
RIFE	VOID
SEAS	CALM
WORD	LETTER
CAUSE	EFFECT
FIGHT	BATTLE
FURRY	WOOLLY
SEDGE	LEDGE
CATTER	CHATTER
MIKING	FILMING
NESTED	SNUGGLED
SEEPING	OOZING
REVERIES	DAYDREAMS
DOCTORESS	DEFIER
PHENOMENA	MIRACLE

And, if you want to analyze, here is a CSV version:

Christmas Words™,Not Christmas Words™
DO,LO
BE,LET
GOD,LAY
HAS,ALL
JOY,OLD
GORY,BLOODY
RIFE,VOID
SEAS,CALM
WORD,LETTER
CAUSE,EFFECT
FIGHT,BATTLE
FURRY,WOOLLY
SEDGE,LEDGE
CATTER,CHATTER
MIKING,FILMING
NESTED,SNUGGLED
SEEPING,OOZING
REVERIES,DAYDREAMS
DOCTORESS,DEFIER
PHENOMENA,MIRACLE

The puzzle satisfies the series' inbuilt assumption, that each word can be tested for whether it is a Christmas Word™ without relying on the other words.
These are not the only examples of Christmas Words™; many more exist.

What is the special rule these words conform to?

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Connect the Clues

Is MANGER a Christmas Word™?
Please explain your answer!

Answer 1

Christmas Words

can have Ls inserted to make Christmas-related words. (Specifically, songs from carols? I think?)

Specifically...

DOLL, BELL, GOLD, HALLS, JOLLY, GLORY, RIFLE, SEALS, WORLD, CLAUSE, FLIGHT, FLURRY, SLEDGE, CLATTER, MILKING, NESTLED, SLEEPING, REVELRIES, DOCTORLESS, PHENOMENAL

Answer 2

Christmas Words

Have no L (noel), but can have one (or more; specifically: two, consecutive) inserted or appended to become a new word

so MANGER

is indeed a Christmas word, because we can insert an L to make it MANGLER

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Edit: This is wrong (though it seems to be close, so I'll leave it here) because DEFIER is a Not Christmas Word™