A name for a day and a season
In present-day English, Christmas is the festival that commemorates the birth of Jesus, kept in most Western churches on 25 December. It is also the stretch of days around that date, filled with leave from work, gifts, and meals. The same form is used attributively in a long list of household compounds, and more loosely as a mild cry of surprise. The origin of the word christmas is English through and through: two older words joined on English soil, not a single label borrowed from French or Latin ready-made.
An English compound of the late Old English period
Late Old English has Cristes mæsse, also written as one item Cristesmæsse. The first element is the genitive of Crist; the second is mæsse, the eucharistic service and, by extension, the feast that held it. Standard dictionaries place the type before the twelfth century. Surviving texts do not show the compound much earlier than about the year 1000. One of the earliest uses is in a short piece associated with Wulfstan, Archbishop of York, who died in 1023, listing Christmas with Candlemas and Easter as a time when church dues were due. Later entries in the Anglo-Saxon Chronicle record kings holding court at Cristesmæsse. A date of 1038 is often repeated as the first record; the major historical dictionaries are more cautious and simply assign the word to the Old English period. The compound may be older than the manuscripts that happen to survive.
Middle English inherited a crowd of spellings: Cristemasse, Criste-mas, Cristesmasse, and others. From the middle of the fourteenth century the two halves are often written as a single word. Historical dictionaries treat the modern name as formed within English by compounding. They also leave room for a second, less certain path: some later forms may reflect a shortening of Christenmas, a related name built on Christen rather than Christ. That extra route is offered as a possibility, not as a closed case.
Neighboring Germanic languages built similar phrases independently. Middle Dutch has kerstmesse, later Dutch Kerstmis; Middle Low German has kristmisse and kerstmisse; Middle High German has kristesmesse, later German Christmesse. West Frisian Krystmis and Saterland Frisian Kristmisse belong to the same pattern. These are cognate formations, not the parents of the English word.
Two halves: the anointed and the dismissal
Crist came into Old English from Latin Christus, from Greek khristos, a title meaning the anointed one and used to render Hebrew mashiach. Historical accounts of English pronunciation link the long vowel in the first syllable to Irish missionary work in England in the seventh and eighth centuries. The ch- spelling, now fixed, was rare before about 1500. Capitalization of the name was not settled until the seventeenth century. Latin Christus also displaced an older English title, Hæland, healer or savior, as the usual designation for Jesus.
Mass in the liturgical sense is Old English mæsse, from ecclesiastical Latin missa. That Latin noun is generally taken as a dismissal, from the verb mittere, to send or let go, because the rite ended by sending the congregation out. The explanation is widely held but not beyond dispute. In English the same second element later behaves as a suffix -mas on other feast names: Candlemas, Michaelmas, Martinmas, Lammas. Christmas remains transparent: Christ plus that holiday ending.
English already had winter names before this Christian compound was common. Geol, later Yule, and midwinter marked the season. Christmas did not wipe those words out; it supplied the specifically ecclesiastical title for the nativity feast.
From the mass of a day to a household calendar
The earliest sense is the liturgical festival itself and the day that held it. By Middle English the name also covers the eve of the feast — Cristenmesse Even is recorded around 1300 — and the broader season of eating and gathering. A verb meaning to keep or celebrate Christmas appears in the 1590s. Father Christmas is named in a fifteenth-century carol linked with Richard Smart, rector of Plymtree in Devon.
Later compounds follow the feast into domestic and commercial life. Christmas present is on record from 1769. Christmas tree in the modern sense is American English by 1835, rendering German Weihnachtsbaum. Designed cards appear in 1843, and the phrase Christmas-card is in use by 1850. As an exclamation the word is attested by the mid-eighteenth century. These later senses do not rewrite the etymology; they show a church-calendar name stretching over gifts, evergreens, and printed greetings.
The spelling Xmas is part of the same history, not a modern attempt to strip Christ from the word. Medieval scribes already shortened Christ with the Greek letter chi, written as X, sometimes with rho as XP. An early twelfth-century copy of the Peterborough Chronicle, in the annal for 1101, has xpes mæssan for the same feast. Early modern English shows X'tenmas in 1551 and Xtmasse in 1660; Xmas itself is cited from 1721.
A Pacific recasting
One later descendant is easy to miss on a map. In 1777 James Cook reached a Pacific atoll on Christmas Eve and put Christmas Island on the English chart. In Gilbertese, the language of Kiribati, the English holiday name was rewritten as Kiritimati: the language has no letter s of that kind, and the sequence ti stands for the s-sound, so the local spelling still sounds close to the English word. Dictionaries treat Kiritimati as a doublet of Christmas. A compound coined in late Old English for a winter mass thus traveled, through a navigator's calendar and a later spelling system, into a toponym that no longer looks English at all.