The Roman calendar originally had ten months and September, October, November, and December are the Latin words for seven, eight, nine and ten. Roman emperor Numa Pompilius then inserted two more months, January and February at the beginning and all the other months shifted two places.
January: Janus, Roman god of doors, beginnings, sunset and sunrise, had one face looking forward and one backward
February: On February 15 the Romans celebrated the festival of forgiveness for sins; (februare, Latin to purify)
March: Mars, the Roman god of war
April: Roman month Aprilis, perhaps derived from aperire, (Latin to open, as in opening buds and blossoms) or perhaps from Aphrodite, original Greek name of Venus
May: Maia, Roman goddess, mother of Mercury by Jupiter and daughter of Atlas
June: Juno, chief Roman goddess
July: Renamed for Julius Caesar in 44 BC, who was born this month; Quintilis, Latin for fifth month, was the former name (the Roman year began in March rather than January)
August: Formerly Sextilis (sixth month in the Roman calendar); re-named in 8 BC for Augustus Caesar
September: September, (septem, Latin for 7) the seventh month in the Julian or Roman calendar, established in the reign of Julius Caesar
October: Eighth month (octo, Latin for 8) in the Julian (Roman) calendar. The Gregorian calendar instituted by Pope Gregory XIII established January as the first month of the year
November: Ninth Roman month (novem, Latin for 9). Catholic countries adopted the Gregorian calendar in 1582, skipping 10 days that October, correcting for too many leap years
December: Julian (Roman) year's tenth month (decem, Latin for 10).