Christmas is December 25th. Christmas Eve is December 24th. Some people think that December 23rd should be called “Christmas Adam” because Adam came before Eve.
 
“Christmas Adam” has been cited in print since at least 1867.
 
“New Year’s Adam” (December 30, the day before New Year’s Eve) is a similar term.
 
           
Google Books 
December 1867, Scott’s Monthly Magazine, pg. 953:
CHRISTMAS ADAM.—A little girl came to her mother on the 23d of December, and asked: “Mamma, is to-day Christmas Adam?”  “No, my daughter,” was the reply, “but why do you ask?” “Because to-morrow is Christmas Eve, and I thought to-day would be Christmas Adam.
   
Newspapers.com
8 January 1871, Chicago (IL) Tribune, “Tit-Bits,” Supplement, pg. 5, col. 8:
Quite a young Sunday School pupil, whose Bible readings had not extended beyond Genesis, and who participated in the festivities of Christmas Eve, and also in those of Monday night, asked her teacher if the latter would be called “Christmas Adam!”
 
Google Books
December 1896, Judge’s Library (New York, NY), pg. ?, col. 2:
UNFAIR.
The women now are so preferred
It is becoming quite absurd.
Of Christmas Eve so much is heard—
Of Christmas Adam not a word.
     
Google Books
2 November 1900, Harvard Lampoon, pg. 111, col. 2:
IF Christmas Eve ‘s the night before,
Oh, when is Christmas Adam?
   
Google Books
Love’s Tender Path
By Birdie L. Etchison
Ukrichsville, OH: Heartsong Presents
1996
Pg. 97:
“If December twenty-fourth is Christmas Eve, then the twenty-third should be Christmas Adam,” she had said, laughing. “Didn’t Adam come before Eve?”
 
Google Groups: alt.arts.limericks
Anticipation
Liam na Beag
12/31/01
Since Adam was ere Eve’d had ‘im
A precedent t’honor, she bade ‘im
  For it is, I perceive
  If the day before’s “Eve,”
That the one before that is “Adam.”
 
“Christmas Adam” is Dec. 23rn,
“Christmas Eve,” then, is Dec. 24rn.
  The next day, my fear is
  If your gifts aren’t too dear, is
Often label-ed “Christmas Return.”
—
Liam “Ever the wordsmith” na Beag
 
Google Groups: rec.crafts.textiles.yarn
OT: It’s the Eve of Christmas Eve….
Ranee Mueller
12/23/04
(...)
We call it Christmas Adam, as Adam came before Eve.
     
Google Groups: uk.food+drink.misc
Christmas Lunch/Dinner
Gregoire Kretz
12/7/05
Ophelia wrote:
> “Wayne Boatwright”

wrote in message
> news:Xns9725433388D17waynesgang@217.22.228.19…
> > On Wed 07 Dec 2005 01:08:04a, Thus Spake Zarathustra, or was it
> > Ophelia?
> >
> >> What is a Christmas Adam?
> >
> > Christmas Adam comes before Christmas Eve.  Seriously.  Many people
> > celebrate it at a quiet dinner on December 23rd.
 
You mean, it really exists? Funny, I was just making it up, to give Eve a break for a change. 😊
 
Urban Dictionary
Christmas Adam
The day before Christmas Eve. Since Adam was created before Eve, and the need to be equitable in the holiday season, Christmas Adam creates further anticipation of Christmas Day.
(...)
#christmas#holiday#celebration#adam#eve
by JSR December 23, 2005
 
Urban Dictionary
Christmas Adam
The day before Christmas Eve (since Adam came before Eve)
Today is December 23 - it’s Christmas Adam.
#christmas eve#christams eve eve#christmas#holiday#seasonal#
by BassInYourFace December 23, 2009
     
Reddit—Jokes
Posted by u/bag-pipes December 15, 2015
What do you call the day before Christmas Eve?
Christmas Adam
COMMENTS
AdmiralDan75
Festivus
     
Reddit—Logophilia
Posted by u/bruce656 December 20, 2016
Is there a name for the day before Christmas Eve, December 23rd?
An actual name, not something silly like Christmas Eve Eve or Christmas Adam. I tried posting to r/whatstheword, but didn’t get much help there.
COMMENTS
PostHedge_Hedgehog
It’s been called Little Christmas, or “lillejul”, in Sweden for centuries https://sv.m.wikipedia.org/wiki/Lilla_julafton?wprov=sfla1
ruuuhhy
My Jewish friends and I call it Erev Christmas Eve. 
   
Google Books
His Frontier Christmas Family
By Regina Scott
Harlequin
2017
Pg. ?:
It was family custom to call the twenty-third of December Christmas Adam, because it was the day before Christmas Eve, and Adam had come before Eve.
 
Reddit—Dadjokes
Posted by u/AnitaRide December 23, 2019
Did you know December 23 is a holiday too?
Its Christmas Adam. Because Adam came before Eve