January
Old English, from Latin Januarius (mensis ) ‘(month) of Janus’ (see Janus), the Roman god who presided over doors and beginnings.
wiktionary
From Middle English Januarie, januari, re-latinised forms of Middle English Janevere, Ieneuer, from Anglo-Norman genever, from Latin iānuārius(“(month) of Janus”), perhaps from a Proto-Indo-European *h₁ey-(“to go”, a root). Doublet of Gennaro.
etymonline
January (n.)
late 13c., Ieneuer (early 12c. in Anglo-French), from Old North French Genever, Old French Jenvier (Modern French Janvier), from Latin Ianuarius (mensis) "(the month) of Janus" (q.v.), to whom the month was sacred as the beginning of the year according to later Roman reckoning (cognates: Italian Gennaio, Provençal Genovier, Spanish Enero, 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.
