Use of the word Xmas
1767  The Universal Spelling-Book, Originally Published: London : Sold wholesale by G. & H. Davidson ... , [1767], p. 54. 
“Table XIX.
Of Contractions of such things as are necessary to be understood, in which whole words and sentences are known by certain Letters only.”
“J.H.S. Jesus, Saviour of Men
"Xmas. Christmas
"Xn. Christian"

1775 From the Library of Congress site:  American Memory.
Samuel Chase to John Adams
“A Dutch Vessell bound to Surrinam has contracted with a Captain of this place for twenty five Tons, if he comes for it by Xmas.”

1776 From the Library of Congress site: American Memory.
Letters of Delegates to Congress: Vol: 3 January 1, 1776 – May 15, 1776
Francis Lightfoot Lee to Landon Carter
“…I hope Ld. Dunmore has left you spirits enough, Still to enjoy a little Xmas mirth, and to give him a warm reception, if he shoud venture up Rapa...”

1778 From the Library of Congress site: American Memory.
Letters of Delegates to Congress: Vol: 11 October 1, 1778 – January 31, 1779 John Williams to Robert Burton.
“My love to Magg & the two little boys and tell them I intend to Eat My Xmas dinner with them at furthest for I assure you I am heartily wearied of this place.”

1839  Southern Literary Messenger, Feb. 1839.
an article about a man who became acquainted with Patrick Henry's elderly daughter, and got to see some of Patrick Henry's letters that she'd saved. In describing them, he wrote,
"The word Christmas he spells Xmas."

1851  From Encyclopaedia Americana, 1851:
"Abbreviations in common use.... Xmas.Christmas. Xn. Christian."

1856  From Wright American Fiction 1851-1875 site.  Edith Hale by Thrace Talmon, Boston: Phillips, Sampton, 1856, p. 171.
“‘I shall now ask you some questions in primary instruction,’ said Mr. Crabtree. ‘ Among the abbreviations are some of which I find many teachers ignorant, but which must be known and taught, to attain the true standard…’
…“Xmas?” pursued Mr. Crabtree.  Edith could not tell; and she began to feel very much confused.
“It is an abbreviation of Christmas,” said Mr. Crabtree.
“Xn?” Edith was at a loss again.
“Means Christian.
“‘I did not know as the word Christian was ever abbreviated,’ ventured Edith.
“‘Neither did I,’ said Mr. Swinton, smiling, ‘only in cases where the word is used to designate particular graces.’
“‘It is upon the list of abbreviations in common use,’ said Mr. Crabtree; ‘and I might ask many more, which, I presume, you could not tell,’ he added.”

1860  From abbreviations listed in Worcester's "An Elementary Dictionary of the English Language," 1860:
"Xmas., or Xm. Christmas
Xn., or Xtian. Christian.
Xnty., or Xty. Christianity...
Xt. Christ."

1860 Valley of the Shadow Project J.D. Imboden Staunton Dec. 3, 1860
“Major McCue is in S. Carolina to furnish them guns--His new gun is a complete success--Hurry up Dr Coleman with that Rice fee--I will be awfully hard up by Christmas--I have already sent $500 to Ky-must send $1000 more at Xmas--& the same amt 1st Feby.”

1860  Diary entry From Documenting the American South Dec. 29, 1860
"The Xmas passed very pleasantly with us, the boys were all here and we had a tree fixed off in a very lively and interesting manner "

1861 Valley of the Shadow Project Alexander H.H. Stuart to Frances Peyton Suart, Richmond. Jan. xx, 1861.
“He is a decided secessionist -- He says he spent Xmas at Mr Towles's & that Mr Towles has been elected a secession delegate to the Louisiana Convention –“

1862  Valley of the Shadow Project. Addie Garber to Thomas Garber, Staunton. January 19, 1862.
“Tommie if you ever want those three dollars I asked you for Xmas send me word and you shall have them.”

1863  Valley of the Shadow Project. Jedediah Hotchkiss to Sara A. Hotchkiss, Morton's House December 3, 1863.
“I have not yet made my application for a leave at Xmas, but do not doubt I shall be able to get one”

1863  Jacob Kent Langhorne to His Parents, Virginia Mil Institute, January 2, 1863Virginia Mil Institute
“I cannot express my thanks in writing for the nice box you sent me also for the 10$ you sent me for Xmas gift…
We only had one day Christmas and I was very glad of it. I thought of home so that I did not enjoy my self Xmas day.”

1865  Diary entry From Documenting the American South Jan. 16, 1865
"We passed a sad Xmas..." 
"We had many mercies, we were well, & the family at home enjoyed a bountiful repast, on Xmas day and on New Years day."

1865  Elliott and Gonzales Family Papers Manning Feby 2d 1865
"I feared his regiment was drawn into the fighting near Wilmington and have not heard from him since my last to him written before you saw me at Xmas."

1868 From the Brooklyn Daily Eagle December 24, 1868
“Atheneum, Brooklyn
TO-NIGHT, LAST TIME OF LINGARD.
TO-NIGHT, LAST TIME OF LINGARD.
TO-NIGHT, LAST TIME OF LINGARD.
TO-NIGHT, LAST TIME OF LINGARD.
TO-NIGHT, LAST TIME OF LINGARD.
TO-NIGHT, LAST TIME OF LINGARD.
THIS XMAS EVE.
THIS XMAS EVE.
THIS XMAS EVE.
THIS XMAS EVE.
POSITIVELY HIS LAST APPEARANCE.”