Since the dawn of time, mankind has felt the need to understand the cycles marking the rhythmic passing of the seasons and the changing length of day and night, and to measure the flow of time. To answer these questions of crucial importance for agricultural activities, religious practices and social and economic life, human beings attentively observed the periodic motions of the great celestial bodies sweeping through the sky.

The most ancient calendars were in fact based on the cycles of the Sun and the Moon.
In the Mediterranean civilisations, the tendency to base the calendar on the lunar cycle prevailed at first. Our satellite completes one entire lunation in 29 and a half days. Accordingly, in a solar year there are 12 lunations, making a total of 354 days; 11 days less than in the solar year.
The lunar cycle is not synchronised with the passing of the seasons, which is determined by the course of the Earth around the Sun and by the inclination of the Earth's axis by 23 and a half degrees in relation to the ecliptic.
The Greek astronomer Meton noted however that 235 lunations corresponded almost exactly to 19 solar years. With the Metonic cycle, the lunar calendar could be synchronised with the solar year and with the changing of the seasons.

The ancient peoples of Mesopotamia adopted a lunisolar calendar in which the beginning of the month was established by the appearance of the first crescent moon after the new moon. The lunar calendar was kept in step with the solar one by the addition, established by the king, of an intercalated month.

The Egyptitans, as Herodotus relates, utilised a calendar based on the apparent course of the Sun
It lasted exactly 365 days, made up of 12 months of 30 days each, to which were added 5 days at the beginning of each year. The element that governed the civil and religious calendar was the reappearance of the star Sirius (identified with the goddess Isis), in the constellation of Canis Major, which announced the annual inundation of the Nile. Sirius becomes visible again on the eastern horizon, just before sunrise, around July 20th, after which it remains invisible for about 70 days. The heliacal rising of Sirius and the changing position of the Sun at dawn during the course of the year were observed from fixed reference points, such as obelisks. The Egyptian solar year was shorter than the true solar year by about six hours, resulting in a discrepancy of one day every four years.

Legend has it that Romolus, the first king of Rome, established the most ancient Roman calendar, which may have had only 10 months, as seems indicated by the list of their names. March, April, May and June were in fact followed by Quintilis, the 5th month, Sextilis, the 6th, and then September, the 7th month, and so on. The months lasted 30 days, except for March, May, Quintilis and October, which lasted 31 days, for a total of only 304 days, a duration so short as to cast doubt on whether Romulus' calendar ever really existed.

Numa Pompilius, the successor to Romolus, is traditionally attributed with having enacted the first reform of the calendar, borrowed from the lunar calendar of the Greeks, which had a year 354 days long (equivalent to 12 lunations of 29 and a half days each), obtained by alternating months of 29 days with months of 30 days. To make the lunar calendar coincide with the solar year, every other year a month called Mercedonius, lasting alternatively 22 and 23 days, was intercalated. Mercedonius was inserted after February 23rd, the day of the Saturnalia festival.

But the Collegium Pontificum, the ancient Roman body that presided over religious matters and management of the calendar, manipulated the intercalated months for political purposes, creating discrepancies between the solar year and the civil year. It sometimes happened, for instance, that spring festivals were celebrated in mid-autumn.

In 46 B.C., Julius Caesar enacted the reform of the calendar that bears his name, established with the consultation of the Alexandrine astronomer Sosigenes. The Julian reform marked the transition to a solar calendar. The average duration of the solar year was established as 365 ¼ days, obtained through a four-year cycle consisting of three years lasting 365 years and one lasting 366. For this purpose the reform also established for the months of January, August and December a duration of 31 days, while one day each was added to the remaining months of 29 days. The extra day in the fourth year of the cycle was obtained by duplicating the 24th of February, the sixth day before the calends of March, which was called 'bis sextus', that is, sixth day repeated. From this came the term 'bissextile year', also known as 'leap year'.

In 44 B.C. Mark Anthony changed the name of the month Quintilis to Julius, the July of today, in honour of Julius Caesar. In 8 A.D. the month Sextilis was renamed Augustus, today's August, in honour of Octavian. In the Julian calendar, the year still had a discrepancy of about 11 minutes in relation to the actual duration of the solar year. Consequently, in the 16th century, the date of the spring equinox was moved 10 days away from the occurrence of the astronomical event.

In the High Renaissance, Pope Gregory XIII appointed a special Calendar Reform Committee.
After long discussion it was agreed to accept the proposal of the Calabrian physician Luigi Lilio to consider bissextile only those years that were multiples of 100 divisible by 400. Acccordingly, in the last four centuries, the years 1700, 1800 and 1900 were not bissextile years.
To eliminate the discrepancy of 10 days accumulated during the 15 centuries that had elapsed since the introduction of the Julian calendar, it was decided that Thursday October 4th, 1582, would be followed by Friday, October 15th. The spring equinox thus fell once again on March 21st, the date established by the Council of Nicea in 325 A.D. The Council had also decided that the moveable feast of Easter was to be celebrated on the Sunday following the first full moon after the spring equinox. The Gregorian reform thus re-established precise coincidence between the astronomical event and the religious holiday.
The Gregorian calendar was promulgated, by Papal bull, in 1582. At first adopted only in the Catholic world, it progressively spread to the rest of the planet and is still today almost universally employed