The names of the months are Roman
Among the many things that the Romans have left us no less important is the calendar with the names of the months, days and seasons. To understand the logic and coherence of the names "september= September; october= October; november= November and december= December," which etymologically means "seventh, eighth, ninth and tenth" we must know that the primitive Roman year had ten months and began in March.
It is attributed to the mythical king Romulus the foundation of Rome in 753 BC and the establishment of the first solar calendar of 304 days a year. The same tradition attributes to King Numa, who ruled from 715 to 673 BC, a reform that probably corresponds to that made in the mid-fifth century BC and mainly consisted of adding two more months, thus forming a twelve month calendar, which take their name from gods and religious holidays:
Ianuarius, (January) dedicated to Janus, god of gates and beginnings and also of the beginning of the year on the crescent moon after the winter solstice on December 25th.
Februarius, (February) month in which they celebrated the festival of purification called "februa", Etruscan word which literally means purification.
Martius (March), dedicated to the god of war Mars. In this month began the mandate of the consuls and military campaigns.
Aprilis (April), word that some authors relate to aperire = open, to refer to the moment when nature, in spring, blooms. Others consider it has an Etruscan origin, taken from the Greek and related to Aphro-dite or Aphrodite, the goddess of love.
Maius (May) that takes its name from Maia, the goddess of growth.
Iunius (June) in honor of the goddess Juno, wife of Jupiter, the goddess of marriage among many other things.
Quintilis (fifth month), when the year began in March, then called Iulius (July) in honor of Julius Caesar after his violent death.
Sextilis (sixth month), later renamed Augustus (August) in honor of Octavian Augustus when he was still alive.
September (seventh month; September). Because in Latin seven is said “septem”.
October (eighth month; October). Because in Latin eight is said “octo”.
November (ninth month; November). Because in Latin nine is said “novem”.
December (tenth month; December). Because in Latin ten is said “decem”.
The year began at first in March, what explains, as we´ve already said, the names of the months from September to December. In March it also began the mandate of the consuls and military campaigns of the Romans by the Lazio and Italy. But when the wars had to be taken out of Italy, the beginning of the campaign in March turned out a serious drawback due to the time it took the army to move to the point of conflict.
This was evidenced in the Celtiberian Wars in Hispania. Given the great distance, the Senate moved the beginning of the consular year 153 BC to January 1st so that the consul Quintus Fulvius Nobilior had time to go to Hispania, where incidentally he was defeated by the Celtiberian tribes of the Belli, Titii and Arevaci in August of that year.
Julius Caesar was in Egypt with the famous Queen Cleopatra, who apparently he had his son Caesarion with. There he met the solar calendar and realized about the benefits of its implementation in Rome.
At his death it was given his name, Julius, to which in the primitive calendar was Quintilis, the fifth month. Then the next month was called Augustus in honor and life still of Octavian Augustus. Other months were also called after the name of any other emperor, but this nomination had no lasting success in any other case.
Every month hadn´t then the same length, rating from the 28 or 29 days of February to the 30 days which have November, April, June and September and the 31 days of January, March, May, July, August, October and December. This disparity, at first due to different festivals, taboos and religious superstitions, goes on to our present days.
As it happens with the days of the week, the names of the months have lasted in the Latin languages and have also been imposed in the countries of Saxon and Germanic languages. The intense Christianization of pagan beliefs and customs could not beat either the pagan origin or name of these twelve periods into which the year is divided.