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late 13c., Ieneuer, from Old North French Genever, Old French Jenvier (Modern French Janvier), attested from early 12c. in Anglo-French, from Latin Ianuarius (mensis) "(the month) of Janus," to whom the month was sacred as the beginning of the year (see Janus; cf. Italian Gennajo, Provençal Genovier, Portuguese Janeiro). The form was gradually Latinized by c.1400. Replaced Old English geola se æfterra "Later Yule." In Chaucer, a type-name for an old man.
first month of the Gregorian calendar. It was named after Janus, the Roman god of all beginnings. January replaced March as the first month of the Roman year no later than 153 BCE.