Cristes mæsse

The earliest written form of the word "Christmas" is found throughout an Anglo-Saxon Chronicle  manuscript dated at approximately 1123. For example:

an 1101 Her on isum eare to Xes. mæssan heold se cyng Heanri his hired on Westmynstre.
[year 1101 In this year at Christ's mass King Henry held his court in Westsminster]

About the characters in the OE quote:

The first rune is "th" or thorn, (as in "this")
The second rune is a "y"or yoch (as in "year")

The next set of letters are Greek, not AS -- chi rho for Christ with the Anglo-Saxon genitive ending -es

Then there's another "y" at the end of Henry.

The illustration is a "detail and initial 'B' from a page from the Anglo-Saxon Chronicle, in Anglo-Saxon. Made in Peterborough Abbey (12th century, c. 1121). MS. Laud Misc. 636, fol. 1r. © Bodleian Library, Oxford."

Comments

[gail]

The first rune is "th" or thorn, (as in "this")
The second rune is a "y"or yoch (as in "year")

The next set of letters are Greek, not AS -- chi rho for Christ with an AS genitive ending -es

Then there's another "y" at the end of Henry.

[Minnesotastan]

I also found it in 1038. (presumably the quote from 1101 was meant to be illustrative and not the actual first mention).

[gail]

Sorry that was unclear. There are references to various earlier dates, but it was all written down in approximately 1123 (date given by the OED). I would assume it refers to the Peterborough ms.