What is the X in X-mas?
late 14c., from Old French preface "opening part of sung devotions" (14c.) and directly from Medieval Latin prefatia, from Latin praefationem (nominative praefatio) "fore-speaking, introduction," in Medieval Latin "prologue," noun of action from past participle stem of praefari "to say beforehand," from prae "before" (see pre-) + fari "speak" (see fame (n.)).
1610s, from preface (n.). Related: Prefaced; prefacing.