The early ancient Roman calendar was made of ten months only. This made November and December the ninth and tenth months in the year. Being practical the Romans called these months by their number: novem and decem hence november and december. By the time of Julius Caesar an extra couple of months had been stuck on to the beginning of the calendar.
This image, taken from an 18th century book of dates, numbers, inscriptions and astronomical events was printed in Paris, France in 1763. Title: “Tablettes Chronologiques de l’Histoire Universelle, Sacree et Prophane, Ecclesiastique et Civile, Depuis la Creation du Monde justqu’a l’an 1762.” by Abbot Lenglet Dufresnoy. Book 1 , Ancient History.
| January and February | March and April | May and June | July and August | September and October | November and December |