Why are chritmas called :x'max"

the short-form of christmas is X'MAS.

from english dictionary, chinese meaning of X'MAS is
n.
耶誕節(=Christmas)

"Xmas" and "X-mas" are common abbreviations of the word "Christmas". They are sometimes pronounced "eksmas", but they, and variants such as "Xtemass", originated as handwriting abbreviations for the pronunciation "Christmas". The "-mas" part came from the Anglo-Saxon for "festival", "religious event": Crīstesmæsse or Crīstemæsse. This abbreviation is widely used but not universally accepted; some view it as demeaning to Christ, whilst others find it a helpful abbreviation.
Usage of X for Christ
The word "Christ" and its compounds, including "Christmas", have been abbreviated for at least the past 1,000 years, long before the modern "Xmas" was commonly used. "Christ" was often written as "XP" or "Xt"; there are references in the Anglo-Saxon Chronicle as far back as 1021 AD. This X and P arose as the uppercase forms of the Greek letters χ and ρ), used in ancient abbreviations for Χριστος (Greek for "Christ"), and are still widely seen in many Eastern Orthodox icons depicting Jesus Christ. The labarum, an amalgamation of the two Greek letters rendered as ☧, is a symbol often used to represent Christ in Catholic, Protestant, and Orthodox Christian Churches.
Some believe that the term is part of an effort to "take Christ out of Christmas" or to literally "cross out Christ"; it is also seen as evidence of the secularization of Christmas, as a symptom of the commercialization of the holiday (as the abbreviation has long been used by retailers). It may also be used as a vehicle to be more inclusive (See political correctness).


圖片參考：http://upload.wikimedia.org/wikipedia/commons/thumb/7/7b/Simple_Labarum2.svg/150px-Simple_Labarum2.svg.png



圖片參考：http://en.wikipedia.org/skins-1.5/common/images/magnify-clip.png
The labarum, often called the Chi-Rho, is a Christian symbol representing Christ.
The occasionally held belief that the "X" represents the cross Christ was crucified on has no basis in fact. St Andrew's Cross is X-shaped, but Christ's cross was probably shaped like a T or a †. Indeed, X-as-chi was associated with Christ long before X-as-cross could be, since the cross as a Christian symbol developed later. (The Greek letter Chi Χ stood for "Christ" in the ancient Greek acrostic ΙΧΘΥΣ ichthys.) While some see the spelling of Christmas as Xmas a threat, others see it as a way to honor the martyrs. The use of X as an abbreviation for "cross" in modern abbreviated writing (e.g. "Kings X" for "Kings Cross") may have reinforced this assumption.
In ancient Christian art χ and χρ are abbreviations for Christ's name. In many manuscripts of the New Testament and icons, X is an abbreviation for Christos, as is XC (the first and last letters in Greek, using the lunate sigma); compare IC for Jesus in Greek. The Oxford English Dictionary documents the use of this abbreviation back to 1551, 50 years before the first English colonists arrived in North America and 60 years before the King James Version of the Bible was completed. At the same time, Xian and Xianity were in frequent use as abbreviations of "Christian" and "Christianity"; and nowadays still are sometimes so used, but much less than "Xmas". The proper names containing the name "Christ" other than aforementioned are rarely abbreviated in this way (e.g. Hayden Xensen for the actor name "Hayden Christensen"). This apparent usage of "X" to spell the syllable "kris" (rather than the sounds "ks")has extended to "xtal" for "crystal" and on florist signs ""xant" for "chrysanthemum" (though these words are not etymologically related to "Christ"; "crystal" comes from a Greek word meaning "ice", and "chrysanthemum" from Greek words meaning "golden flower", while "Christ" comes from a Greek word meaning "anointed").