The first month of the modern Gregorian calendar owes its name to a Roman god.

In classical mythology, Janus was the deity of archways (jani), doorways (januae), and in a larger sense, the lord of all beginnings. He was often depicted with two faces, one forward and one backward, symbolizing his dominion over gateways.

When a new month replaced March as the first month in the Roman calendar in 153 B.C., January was named after him, and every year a celebration called the Agonium was held in his honor on January 9.