https://www.almanac.com/how-did-months-get-their-names

March (Martius) was named for Mars, the god of war, because this was the month when active military campaigns resumed. May (Maius) and June (Junius) were also named for goddesses: Maia and Juno. April (Aprilis) is thought to stem from the Latin aperio, meaning “to open”—a reference to the opening buds of springtime. The rest of the months were numbered; their original names in Latin meant the fifth (Quintilis), sixth (Sextilis), seventh (September), eighth (October), ninth (November), and tenth (December) month.

Eventually, January (Januarius) and February (Februarius) were added to the end of the year, giving all 12 months proper names. January was named after Janus, the Roman god of beginnings and transitions. February’s name is believed to stem from Februa, an ancient festival dedicated to ritual springtime cleaning and washing.

Julian Calendar Updates When Julius Caesar became Pontifex Maximus, he reformed the Roman calendar so that the 12 months were based on Earth’s revolutions around the Sun. It was a solar calendar, as we have today. January and February were moved to the front of the year, and leap years were introduced to keep the calendar year lined up with the solar year.

The winter months (January and February) remained a time of reflection, peace, new beginnings, and purification. After Caesar’s death, the month Quintilis was renamed July in honor of Julius Caesar in 44 BC, and later, Sextilis was renamed August in honor of Roman Emperor Augustus in 8 BC.

Of course, all the renaming and reorganizing meant that some of the months’ names no longer agreed with their position in the calendar (September to December, for example). Later emperors tried to name various months after themselves, but those changes did not outlive them!