Many people are concerned about gaining “holiday pounds” over Christmas and New Year’s. A saying has been cited in print since at least 1967:
 
“It’s not what you eat between Christmas and New Year’s that makes you fat—it’s what you eat between New Year’s and Christmas.”
 
That is, it’s not the seven holiday days to be concerned about, but the 358 other days. The saying is not known to have any specific author.
   
“Don’t worry about what you eat between Thanksgiving and Christmas. Worry about what you eat between Christmas and Thanksgiving” is a similar saying.
   
   
9 December 1967, Iowa City (IA) Press-Citizen, “Have You Heard?” by Johanne Nelson Beers, pg. 5, col. 1:
FOR THOSE WHO FRET ABOUT PUTTING ON WEIGHT over the holidays, a reader has this sage observation: “It’s not what you eat between Christmas and New Year’s that makes you fat—it’s WHAT YOU EAT BETWEEN NEW YEAR’S AND CHRISTMAS.
 
5 February 1969, Albuquerque (NM) Journal, “Journal Action Line,” pg. A-16, col. 1:
A. Miss Action Line would like to remind you of what a friend said recently: “It’s not what you eat between Christmas and New Year’s that counts; it’s waht you eat between New Year’s and Christmas.”
 
25 December 1969, Atlanta (GA) Journal and Constitution, “How Did He Know He Sleighs?” by Leo Aikman, pg 26-A, col. 3:
It isn’t what you eat between Christmas and New Year’s that makes you fat; it’s what you eat between New Year’s and Christmas.—Connie Fulks, Buffalo Tex. Press.
 
4 January 1970, Daily World (Opelousas, LA), “Entre Nous” by Frank J. Dietlein, Jr., pg. 26, col. 1:
LONG TIME Southern Pacific railroad employee Sidney Simon, who resides in Lafayette, was remonstrating “Tee” Nash about Tee’s weight problem.
 
Said Sidney, “It is not what you eat beween Christmas and New Years that hurts, It is what you eat between New Years and Christmas that keeps you fat.”
 
24 January 1980, Marengo (IL) Beacon-News, “What’s Cookin’?” by Mary Cay Wesolowski, pg. 6, col. 7:
There’s a phrase going around town that I’m getting credit for. I wish that I could say I thought of it. “It’s not what you eat between CHristmas and New Years that counts. It’s what you eat between New Years and Christmas.” Ain’t it the truth!
 
4 January 1982, Morning Advocate (Baton Rouge, LA), “Smiley Anders’ Baton Rouge,” pg. 1-B, col. 1:
A MESSAGE FOR OUR TIME: A reader says this is an appropriate time for our diet-conscious readers to remember this adage: “It’s not what you eat between Christmas and New Year’s that counts, but what you eat between New Year’s and Christmas.”
 
Google Books
Total Health = Wholeness:
A Body, Mind, and Spirit Manual
By Keith Nemec
Wheaton, IL: Total Health Institute
2000
Pg. 279:
The same principle applies to your diet: It’s not what you eat between Christmas and New Year’s that counts (seven days) but what you eat between New Year’s and Christmas (358 days.)
 
Google Books
The Female Body Breakthrough:
The Revolutionary Strength-Training Plan for Losing Fat and Getting the Body You Want
By Rachel Cosgrove
New York, NY: Rodale
2009
Pg. 113:
There’s a saying, “It isn’t what you eat between Christmas and New Year’s that matters; it’s what you eat between New Year’s and Christmas.”
   
Virgin.com
Pledging to eat well between Christmas and New Year
By Richard Branson
27 December 2016
@richardbranson
(...)
There is a saying that goes around at this time of year. “Don’t worry what you eat between Christmas and New Year.” Actually, it is what you eat between New Year and Christmas that really matters. I’ll be making it one of my resolutions to maintain a healthy diet in 2017, which will put me in good shape to achieve my other goals too.