Janus

"Then wars shall cease and savage times grow mild, / and Remus and Quirinus, brethren twain, / with hoary Faith and Vesta undefiled, / shall give the law. With iron bolt and chain / firm-closed the gates of Janus shall remain. / Within, the Fiend of Discord, high reclined / on horrid arms, unheeded in the fane, / bound with a hundred brazen knots behind, / and grim with gory jaws, his grisly teeth shall grind."

Source: tatoeba (6786685)

January is named after Janus, the Roman god of beginnings and doors.

Source: tatoeba (10148316)

In the ages of victory, as often as the senate decreed some distant conquest, the consul denounced hostilities, by unbarring, in solemn pomp, the gates of the temple of Janus. Domestic war now rendered the admonition superfluous, and the ceremony was superseded by the establishment of a new religion. But the brazen temple of Janus was left standing in the forum; of a size sufficient only to contain the statue of the god, five cubits in height, of a human form, but with two faces, directed to the east and west.

Source: wiktionary

[K]nowing that the minds of the people, rendered ferocious by a military life, would never accommodate themſelves to the practice of theſe [principles of justice, laws, and morals], during the continuance of war, he [Numa Pompilius] reſolved, by a diſuſe of arms, to mollify the fierceneſs of their temper: with this view, he built a temple to Janus, near the foot of the hill Argiletum, which was to notify a ſtate either of war or of peace: when open, it denoted that the ſtate was engaged in war; when ſhut, that there was peace with all the ſurrounding nations.

Source: wiktionary