Starting from the night of December 24th, Christian communities celebrate the birth of Jesus Christ. Numerous gifts, luminous trees as well as family gatherings are just some of the highlights that accompany the celebration of Christmas or what some call "Xmas".

But what is the difference between "Christmas" and "Xmas", and what is the meaning of the "X" in the expression?

Many dictionaries indicate that "Xmas" is the abbreviation for "Christmas" in English. The word "mas" is part of the word "Christmas".

It may be noted that the expression is not used by various parties at the present time, since it is considered an informal word currently, and the official authorities prefer not to use it.

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In short: "Xmas" has links to religion, so some people prefer to use the expression while others prefer to keep the word "Christ" on the "mas" instead of the "X".

This is a detailed explanation from orderofpreachersindependent.org:

"Xmas” is a common abbreviation of the word “Christmas”. The “-mas” part is from the Latin-derived Old English word for “Mass”, while the “X” comes from the Greek letter Chi, which is the first letter of the Greek word Χριστός, translated as “Christ”.

There is a common misconception that the word Xmas is a secular attempt to remove the religious tradition from Christmas by taking the “Christ” out of “Christmas”. While “Xmas” is considered to be an informal abbreviation, and should never be used in formal writing, it is historically correct.

The word “Christ” and its compounds, including “Christmas”, have been abbreviated in English 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 AD 1021.

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.

The Oxford English Dictionary (OED) and the OED Supplement have cited usages of “X-” or “Xp-” for “Christ-” as early as 1485. The terms “Xpian” and “Xtian” have also been used for “Christian”.

The dictionary further cites usage of “Xtianity” for “Christianity” from 1634. According to Merriam-Webster’s Dictionary of English Usage, most of the evidence for these words comes from “educated Englishmen who knew their Greek”.In ancient Christian art, χ and χρ are abbreviations for Christ’s name.

In many manuscripts of the New Testament and icons, Χ is an abbreviation for Χριστος, as is XC (the first and last letters in Greek, using the lunate sigma); compare IC for Jesus in Greek."