Why do people write Xmas?
There is more than one reason why people write "Xmas" instead of "Christmas". One reason is to be found in history.
In the Greek alphabet the letter X, or "Chi", is the first letter in the Greek word for "Christ". During the persecution
of Christians the X was used as a secret sign among the Christians.
In old publications and texts "Xmas" is written instead of "Christmas".
Another reason is a practical one. "Xmas" is written faster than "Christmas", and people are basically lazy. Maybe it is
an invention of the advertising industry. Whatever the reason, "Xmas" is very common today.
*Adapted from a text from http://www.englisch-hilfen.de