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Originally, the ancient Roman calendar, allegedly introduced by Romulus when he founded Rome, had ten months: Martius (called after Mars, the god of war), Aprillis (after the Latin word "aperire", meaning "to open" or perhaps also after the goddess of love), Maius (after Maia, the goddess of growth), Junius (after Iuno, Iupiter's wife, or after "iuvenis", meaning "youth"), Quintilius, Sextilius, Septembris, Octobris, Novembris and Decembris. The Romans didn't like even numbers as they symbolised death, so the months had either 29 or 31 days.

King Numa, Romulus' successor, added two months to the Roman year: Januarius (after Janus, the protector of doorways) and Februarius (after Februalia, which means "care" but also was the time for sacrifices at the end of the year).

Julius Caesar altered the lengths of the months. He also changed the name of Quintilius to Julius and shortened Februarius to let "his" month have 31 days. He also moved the beginning of the year from March 25 to January 1, and introduced the leap day, which returned every four years.

Caesar Augustus renamed Sextilius to Augustus and also took a day from Februarius to make "his" month also have 31 days. he gave the leap day to February.

Because the length of one revolution of the earth around the sun is 365.24219 days, even a leap day - making the average year contain 365.25 days - couldn't make the year equal to the actual solar year - each year is eleven minutes and fourteen seconds longer than the solar year. Over the centuries, this had accumulated to ten days by the year 1582. Pope Gregory therefore skipped October 5 to October 14, 1582. He also changed the rule for leap days: centenary years wouldn't be leap years anymore, except when they were divisible by 400.

In 1793, Napoleon introduced a new calendar, according to which the year consists of 12 months each containing three "décades" (10-day weeks), five additional days and a "jour de la révolution" for leap years. He changed the names of days and months and let the era start in 1793. However, this calendar system only lasted for 13 years and in 1805 was replaced by the Gregorian calendar again, which we still use today.