The month of January was named after which ancient Roman god?

January is the first month of the year in the Julian and Gregorian calendars. The first day of the month is known as New Year's Day. It is, on average, the coldest month of the year within most of the Northern Hemisphere (where it is the second month of winter) and the warmest month of the year within most of the Southern Hemisphere (where it is the second month of summer).

January (in Latin, 'Ianuarius') is named after Janus, the god of beginnings and transitions in Roman mythology. Traditionally, the original Roman calendar consisted of 10 months totaling 304 days, winter being considered a month-less period. Around 713 BC, the semi-mythical successor of Romulus, King Numa Pompilius, is supposed to have added the months of January and February, so that the calendar covered a standard lunar year (354 days).

Various Christian feast dates were used for the New Year in Europe during the Middle Ages, including March 25 (Feast of the Annunciation) and December 25. However, medieval calendars were still displayed in the Roman fashion with twelve columns from January to December. Beginning in the 16th century, European countries began officially making January 1 the start of the New Year once again—sometimes called "Circumcision Style" because this was the date of the Feast of the Circumcision, being the seventh day after December 25.