Well, it definitely wasn’t Simon Cowell, Cheryl Cole or even that Gary Barlow. Here’s what the comprehensive and reliable Oxford Reference Online has to say on the matter!
“In this abbreviated form of the name Christmas, first recorded in the 18c, the initial X represents a Greek chi (= ch), the first letter of the name Christ. It is a convenient shortening commonly used in newspaper headlines, on cards, and in personal letters, but it is better to pronounce it as ‘Christmas’ than as ‘ex-mass’ which some people consider tasteless.”
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