Why Christmas is written as X-Mas?

It is actually because 'criss-cross' is a common expression for a diagonal crossing of something. Since 'cross' means '+', then 'criss' (or, strictly speaking, 'criss cross') means 'x' and so "criss"mas is 'x'mas. A few people today wish to remove the 'Christ' from 'Christmas' and so use the shortened form but originally it was jocular and used as a marketing technique (to make people look twice, perhaps, or simply to make it take up less space on show cards).

i was fairly close