The British Museum explains why we call the months what we do:

• January is named after the Roman god Janus. He had two faces so he could see the future and the past!
• February is named after an ancient Roman festival of purification called Februa.
• March is named after Mars, the Roman god of war. The Roman calendar originally began in March, and the months of January and February were added later, after a calendar reform.
• April takes its name from the Latin word aperire, meaning ‘to open’ (just like flowers do in spring!.) The Romans called the month Aprilis.
• May is named after the Greek goddess Maia.
• June is named after the Roman goddess Juno—the god of marriage and childbirth, and the wife of Jupiter, king of the gods.
• July and August were named after two major figures of the ancient Roman world—the statesman Julius Caesar and Rome’s first emperor, Augustus.
• September, October, November and December are named after Roman numbers 7, 8, 9 and 10—they were originally the seventh, eighth, ninth and tenth months of the Roman year!
• Before July and August were renamed after Roman rulers, they were called Quintilis and Sextilis, meaning fifth and sixth months.