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Christ's mass. A day set aside for contemplation of God's gift to us - His Son. Sometimes (cough, cough) gifts are exchanged among humans as a sort of tribute to the gift we received from God.
Sometimes abbreviated Xmas. In a bunch of languages, the X is pronounced Kh, (the hard C is Christ with a little more throat thrown in), so the 'X' is just an acronym for Christ, same as the 'U' is USA stands for United. Additionally, the X is also a symbol for the cross, which Christ died on for your sins. So Xmas is not