October is the tenth month of the year in the Julian and Gregorian calendars and the sixth of seven months to have a length of 31 days. The eighth month in the old calendar of Romulus c. 750 BC, October retained its name (from Latin and Greek ôctō meaning "eight") after January and February were inserted into the calendar that had originally been created by the Romans. In Ancient Rome, one of three Mundus patet would take place on October 5, Meditrinalia October 11, Augustalia on October 12, October Horse on October 15, and Armilustrium on October 19. These dates do not correspond to the modern Gregorian calendar. Among the Anglo-Saxons, it was known as Winterfylleth (Ƿinterfylleþ), because at this full moon, winter was supposed to begin.

October is commonly associated with the season of spring in parts of the Southern Hemisphere, and autumn in parts of the Northern Hemisphere, where it is the seasonal equivalent to April in the Southern Hemisphere and vice versa.