During the reign of Romulus the 10 months of the year were counted as irrationally: some were of 20 days, others 35, others even more.
The first month was in March, devoted to Mars, the god of war, because with the arrival of good season they gave to the campaigns start military.
The second month in April, was dedicated to Aphrodite or sprouts plants, which sprout precisely this month, according to the Latin word "aperio". May instead was dedicated to Maia, the mother of Hermes, or Mercury, while June was dedicated to Juno or Era.
The fifth month later in March called "quintile," the sixth "sestile" etc.. September, October, November and December were the seventh, eighth, ninth and tenth month, which was the last.

Then the king Numa Pompilio, who wanted a calendar with a meaning more civil and military, began in January ( "januarius" from "janua", which means door, which opens and closes, a symbol of the god Janus) in the first place, February (which means something like "purification", because this month the Romans brought bids to the dead and celebrated the feast of Lupercali) in March to the second and third.

The months following the cycle of the moon, the first day of each month corresponded to new moon (new moon) and was called "calende" hence the name "roadmap". The full moon (full moon), towards the middle of the month, was the day of the "ides", dedicated to Jupiter; among calende and ides fell "none", in the day on which appeared the first quarter.
To indicate a day of the month the Romans counted backward from calende, ninth, ides of the month following; Only in the sixth century. d.C. this method was replaced by numbering forward.

The year based on the cycles of the moon is the shortest of 11 days and fourth in relation to the calendar, so the Roman calendar remained behind the sun and the difference increased from year to year, despite attempts made to correct it and so at the time of Julius Caesar month did not correspond to more effective seasons: eg when the calendar marked in March, wheat was already mature. Only the priests knew the exact duration of the year and used to add suddenly called months interlayer, called "mercedonio" on the basis of the reform of Numa.
To remedy many drawbacks, in 46 BC Caesar entrusted to Sosigene, astronomer of Alexandria, Egypt, the task of modify the calendar.
The fixed duration of 30 days for the months of April, June, September and November, February 28 and 31 for all others. In addition, calculating the duration of the calendar in 365 days and 6 hours, was introduced the leap year, setting that every four years, the month of February had 29 days instead of 28. "Bisestili" because on February 24 (ie the sixth day before calende March) was counted twice (a = twice, sextus = sixth day). It was necessary to add 46 additional three months.
After the death of Caesar also the month "quintile" was named in July, in memory of gens Iulia.
But after the death of Augustus the sestile was called in August.

The Romans also had a week of eight days: After seven days working the peasants had a day of rest to get to market, deal with its affairs etc.. Only in the third century. d.C. decided to adopt the week of seven days. To indicate the years the Romans citavano the names of two consuls in office for that year ( "under the consulate ... and ..."). Towards the end of the republic fixed a starting point for the account of the years, choosing April 21 753 BC date the foundation of Rome. In the sixth century. took a year of birth of Christ.