The Roman calendar, named after Romulus, originally was determined by the cycles of the sun and moon. The seasons were those of the agricultural year, and the year then was ten months long (304 days), beginning in the spring in March and ending in December with the autumn planting (the months of January and February were ignored simply because there was no agricultural work in the winter).
According to Livy, it was Numa Pompilius, the successor to Romulus as the second king of Rome in the seventh century BC, who divided the year into twelve lunar months. The calendar of Numa was 354 days long, to which one additional day was added because of the Roman superstition about even numbers. March, May, Quintilis (later named July), and October had thirty-one days. The other months had twenty-nine days, except for February, which had twenty-eight. All were odd numbers, which was considered a good omen, except February, which was devoted to rites of purification (februa) in preparation for the festivals of the new year.
The publication of a revised calendar by the Decemviri (a council of ten magistrates) c.450 BC as part of the Twelve Tables, Rome's first code, is thought to be the "historical" origin of this calendar. The new year was moved from March first to January first by 153 BC to coincide with the Roman civil year and the day when their consuls were installed. (It was on January first in 390 BC that Rome was said to be captured by the gauls.)
Some scholars believe that it was Numa who moved the beginning of the year to January because Janus was a lover of civil and social unity. Janus, they say, reclaimed men from brutal and savage living. That is why he is shown as having two faces. They represent the state and conditions he lead men out of, and those he lead men into. Numa, they say, wanted to emphasis the qualities of the arts and the studies of peace over those of war, which Mars represented.
The year twelve lunar months fell short of the solar year. The college of pontiffs (from pontifex or "bridge maker") were responsible for regulating the calendars and adding (intercalating) days to synchronize the two. Since the Second Punic War (218-202 BC), the priests were hesitant to make any changes at all. The calendar had been manipulated more for partisan political consideration than to adjust for the seasons.
By the time of Julius Caesar, the two calendars were hopelessly confused. When he returned from his Egyptian campaign in 46 BC, the seasons were about three months off. The harvest was being celebrated long before the crops were in. While in Egypt, during his involvement with Cleopatra, he had learned of a new calendar based on the solar year. Julius Caesar, consequently, ask the Alexandrian astromomer Sosigenes to advise him on how to reform the entire Roman calendar system. Sosigenes suggestion of a 365.25 day tropical (between equinoxes) solar year was accepted. Caesar directed that these changes.
The Julian calendar was put into effect on January 1, 45 BC. The months without thirty-one days had one or two additional days added to correct for the ten days that are lost in the lunar calendar. (Supposedly, January, March, May, Quinctilis, October, and December had 31 days; April, June, Sextilis, September, and November had 30; and February, 29.) The solar year is approximately a quarter day longer than the calendar year. Therefore, a single intercalary day was to be inserted every four years. Holidays were established on the anniversaries of Caesar's victories and, when he was assassinated the next year, The name of the fifth month,Quinctilis, was changed Julius (July) to commemorate his assassination. Although this change in the name of the month was ignored until made legal after the appearance of a comet four months later, which, recounts Cassius Dio, was understood to mean that Caesar had become immortal and taken his place among the stars.
Even then, after his death, the pontiffs mistakeningly adjusted for leap year every three years (having counted inclusively). After AD 8, Augustus omitted intercalations until the days were corrected. After that the Julian calendar finally functioned as intended, with February gaining an extra day every four years. The sixth month, Sextilis, was renamed Augustus (it was in August that Egypt had became part of the Roman empire) and given an extra day from February so that it would have the same number as July.
In AD 567 the Council of Tours changed the start of the new year to the beginning of spring in March. After which the date for the New Year varied. The new year was celebrated for eight days, usually beginning on March 25th and ending on April 1st. Many used April 1st as the start of the new year.
The 365.25 day Julian calendar was off a bit, the actual tropical year being 365.242199 days. The difference amounts to 11 minutes and 14 seconds per year. By 1572 AD the calendar was in a full 10 days off. Pope Gregory XIII issued a "papal bull" and the Jesuit astronomer Christopher Clavius went to work to solve this problem. The length of the year was redefined as 365.2422 days, a difference of 0.0078 days per year from the Julian calendar.
This changed amount of error to 3.12 days every 400 years. Clavius had allowed for this discrepancy and suggested that three out of every four centennial years, which would ordinarily be leap years, should instead be regarded as common years. This lead to the practice that no centennial year could be a leap year unless it was divisible by 400. Following this rule 1700, 1800, and 1900 were common years, but the year 2000 would be a leap year. This Gregorian reform gives us an extremely accurate calendar system.
In the year 1582 AD Pope Gregory XIII proclaimed a reform of the Julian Calendar. This Gregorian calendar re-established January 1 as the beginning of the year. (However, communications were poor in those days when news traveled by foot. Many people did not receive the news for several years. Some people simply refused to accept the new calendar and continued to celebrate the new year on April 1. These "backward" folk were labeled as "fools" by the general populace. They were subject to some ridicule, and were often sent on "fools errands" or were made the butt of other practical jokes. This led to what became known as "April Fool's Day.")
The Gregorian calendar has been referred to as the "new style calendar" and the Julian referred to as the "old style calendar."