why is christmas called xmas

It's actually a very interesting history behind the term "Xmas". The word "Christ" in Greek is written with the letter "Chi" (χ). The letter "Chi" was often used as an abbreviation for "Christ" in early Christianity. The letter "X" was then used as a shorthand symbol for "Chi", and eventually the word "Xmas" was used to represent "Christmas". So, in a way, "Xmas" is a way of abbreviating the word "Christmas" without losing the meaning.