A “Thanksgiving Sermon” has been cited since the 1860s, and begins:
   
“My Friends: Thanksgiving Day comes, by statute, once a year; to the honest man it comes as frequently as the heart of gratitude will allow, which may mean every day, or once in seven days, at least.”
 
It was credited to “Dr. Le Grand.” The address mentions the New York City newspaper New York Ledger (1855-1898).
 
American journalist and editor. Edward Sandford Martin (1856-1939) is often credited for the Thanksgiving lines, but he did not create them.
   
     
Google Books
Beadle’s Dime American Speaker
Mew York, NY: beadle and Adams
1863 (This date may be incorrect.—ed.)
Pg. 73:
A THANKSGIVING SERMON—By Dr. Le Grand.
My Friends : Thanksgiving Day comes, by statute, once a year; to the honest man it comes as frequently as the heart of gratitude will allow, which may mean every day, or once in seven days, at least.
 
Google Books
Sanders’ Union Speaker
By Charles Walton Sanders
New York, NY: Ivison, Blakeman, Taylor & Co.
1864
Pg. 62:
A THANKSGIVING SERMON
DR. LeGRAND.
1. My Friends, — Thanksgiving Day comes, by statute, once a year; to the honest man it comes as frequently as the heart of gratitude will allow, which may mean every day, or once in seven days, at least.
 
17 November 1875, Dixon (IL) Sun, pg. 3, col. 2:
A Thanksgiving Sermon.
BY JOHNNY PLAYFAIR.
Thanksgiving Day comes, by statute, once a year; to the honest man it comes as frequently as the heart of gratitude will allow, which may mean every day, or once in seven days, at least.
 
19 November 1941, The Christian Science Monitor (Boston, MA), “It’s Said of Thanksgiving,” pg. 27, col. 6:
Thanksgiving Day comes, by statute, once a year; to the honest man it comes as frequently as the heart of gratitude will allow, which may mean every day or once in seven days, at least.—Edward Sandford Martin.
 
Google Books
15 Minute Family Traditions & Memories
By Emilie Barnes
Eugene, OR: Harvest House
1995
Pg. 166:
Thanksgiving Day comes, by statute, once a year; to the honest man it comes as frequently as the heart of gratitude will allow, which may mean every day, or at least once in seven days — Edward Sandford Martin
 
Google Books
Uplifting Quotes on Gratitude and Goodness to Show Appreciation
By Amy Neumann
Good Plu Tech LLC
2012
Pg. 40:
Thanksgiving Day comes, by statute, once a year; to the honest person it comes as frequently as the heart of gratitude will allow. ~ Edward Sandford Martin
 
Twitter
Chris Johnson
‏@LFSolutionsLLC
“Thanksgiving Day comes, by statute, once a year; to the honest man it comes as frequently as the heart of gratitude will allow.” -Ed Martin
3:18 PM - 23 Nov 2016