The calendar had its origin in the need of peoples, even those whose cultural level was very primitive, to divide time, mainly for religious purposes and later for civil purposes. This division was based on natural phenomena, mainly astronomical, as we can see if we consider that the most important divisions of the calendar, the year, the month, and the day, correspond, theoretically, to the time it takes the Earth to make one revolution around the Sun, the Moon to make one complete turn around the Earth, and the Earth to make one turn on its own axis.

Calendar derives from the Latin word calendarmm ("calendar") or kalendarim ("lenders' ledger"). The latter meaning is due to the fact that interest on debts was paid on the first day of each month, and this day was called calendae or kalendae.

Our calendar is based on the Roman calendar, which at first was only of ten months, leaving an empty space of sixty days at the end of the year corresponding to the dead season of winter and, later, probably during the Etruscan period, under the reign of Numa, was constituted the calendar of twelve months, whose names were as follows.

Martius - March, the month of Mars, the ancient Roman god of war.

Aprilis - April, the month of the goddess Venus. It is a word of Etruscan origin apru: "April", possibly derived from the Greek "foam", a word that refers to the birth of Aphrodite, the Greek goddess of love (identified with the Roman Venus), from the foam of the sea.

Maíus - May, the month of Maya, the Roman goddess of spring.

Iunius - June, the month of the goddess Juno, patroness, among the ancient Romans, of marriages.

Quintilis - The fifth month, from quintus:" fifth".

Sextilis - The sixth month, from sextus: "sixth".

September - September, the seventh month, from septem: "seven".

October - October, the eighth month, from octo: "eight".

November - November, the ninth month, from novem: "nine".

December - December, the tenth month, from decem: "ten".

Ianuarius - January, the month of Janus, the Roman god of doors: ianuae. From the idea of "entrance", of "beginning", it later became the first month, the month of the beginning of the year.

Februarius - February, the month of the februas, purification feasts that took place in February.

This calendar was modified by Julius Caesar, according to Sosigenes, astronomer of Alexandria, in 46 B.C., by means of a reform called "Julian". The months of the Julian calendar are the ones we still use today, only the names of the months Quintilis and Sextilis were changed to Iulius (July, the month of July, that is, of Julius Caesar, who was born in that same month) and Augustus: Augustus, the month of Augustus, the first Roman emperor.

In the year 153 BC January became the first month of the year and February the second; the months of September, October, November, and December retained their names even though they no longer corresponded to their location in the calendar.