What is the X in X-mas?
early 14c., "narrative, account," verbal noun from reckon (v.). Meaning "a settling of accounts" is from mid-14c.; that of "calculation" is from late 14c. Cf. Dutch rekening "a bill, account, reckoning," Old High German rechenunga, German rechnung, Danish regning "a reckoning, computation." Day of reckoning attested from c.1600.