The story
Christmas is two words fused into one: Christ and mass. Old English called the feast Cristes mæsse, "Christ's mass" — the name of the church service celebrating his birth. In Middle English it became one word, Cristemasse.
Two sounds changed. First, the vowel. The vowel of Christ was long, and it later became the "eye" sound of modern Christ. But inside the compound it was short — and a short vowel did not take part in the Great Vowel Shift. So Christ says "eye" while Christmas says "i" as in kiss.
Second, the T. Squeezed between S and M, it was hard to say and dropped out of speech, just as T fell silent in castle and listen. The spelling kept Christ- whole, so readers can still see the two words inside.
The CH is also a fossil: it is the Latin way of writing the Greek letter chi in Christos, and it has been said as a plain K in English all along.
