From the Latin word novem, “nine,” because this had been the ninth month of the early Roman calendar.
November was originally the 9th month of early versions of the Roman calendar and consisted of 30 days.
It became the 11th month of the year with a length of 29 days when the months of January and February were added.
During the Julian calendar reform, a day was added to November making it 30 days long again.
