The Gregorian calendar we use today has its origins in the old Roman calendar, which had only ten months, each with 30 or 31 days. The first four, Martius, Aprilis, Maias, and Iunius, were named after Mars, Aphrodite, Maia, and Juno – these names should be pretty recognizable. After that, I guess the Romans ran out of ideas, so they named the remaining six months Mensis Quintilis, Mensis Sextilis, Mensis September, Mensis October, Mensis November, and Mensis December, which meant “Fifth Month”, “Sixth Month”, and so on. After a while, the ten-month calendar became an issue because all the seasonal holidays shifted to other seasons, so in 46 BCE Julius Caesar stepped in and ordered two extra months added to the beginning of the year (ignoring the numbering system entirely). Those months were Ianuarius, named after the god Janus, and Februarius, named after Februa, a pastoral festival meant to cleanse Rome of evil spirits. Later, Mensis Quintilis and Mensis Sextilis were renamed Iulius and Augustus to honor Caesar and his successor, but months “seven” through “ten” stuck, just as our ninth through twelfth months.
