- January, 31 days
- February, 28 days (29 in leap years, which occurred every four years)
- March, 31 days
- April, 30 days
- May, 31 days
- June, 30 days
- July, 31 days
- August, 31 days
- September, 30 days
- October, 31 days
- November, 30 days
- December, 31 days
Julius Caesar was elected Pontifex Maximus in Rome in 63 BC (or in Roman terminology, "in the consulship of Marcus Tullius Cicero and Gaius Antonius Hybrida"). One of the duties of this office was the regulation of the Roman calendar. The old Roman calendar was an attempt at a luni-solar reckoning, but one that required the semi-regular insertion, or intercalation, of an additional month between February and March, at the order of the Pontifex Maximus. Previous Pontifices Maximi had attended to this duty with varying degrees of faithfulness, but Caesar's immediate predecessor as Pontifex Maximus (Quintus Caecilius Metellus Pius) neglected this duty during his term, and perhaps allowed political considerations to dictate his consistent refusal to perform the required intercalation. The result was that by the mid first century BC the calendar was in complete disarray, with months occurring some 90 days before they were supposed to in seasonal terms.
In his first meetings with the College of Pontifices, Caesar attempted to insert a one-time intercalation of 100 days to bring the calendar back into alignment with the seasons. His colleagues rejected this proposal. Thereafter Caesar inserted the required intercalary month every year while he remained in Rome. But when Caesar left Rome to embark on his administration of his provinces, and stayed away to fight his Gallic Wars, he could not keep up with the calendar at all times. The problem became especially acute after he famously crossed the Rubicon River and started the first of Rome's Civil Wars.
Finally Julius Caesar's travels led him to Egypt, where he met an astrologer named Sosigenes who was quite learned in the wealth of observations that the Egyptians had made of the positions of the sun at various times in the year. Caesar employed Sosigenes to help him devise a completely new system. Although he retained the old Roman month names, he added extra days to most of them and abandoned any further attempts to synchronize months with the phases of the moon. Sosigenes, who had a good notion of the true length of the year (which was actually a few seconds longer in those days than it is now), calculated that an extra day should be added every four years because he believed that a full year lasted 365¼ days.
When Caesar returned to Rome at the end of his wars with the last holdouts against him, he assumed the office of Dictator perpetuus (Dictator for life) and ordered his calendar put into place. He first decreed an intercalary period of 67 days between December of one consulship and January of the next (45 to 44 BC). After that, his calendar was to run without fail and, he hoped, require no further adjustment. After Caesar was assassinated in 44 BC, the Romans mistakenly added a leap year every three years instead of every four, but the Emperor Augustus corrected this mistake and from 1 AD the calendar ran smoothly.
By the Middle Ages it had become apparent that the calendar was not precisely accurate. Caesar had arranged it so that the winter solstice occurred on December 25, the traditional date of the rebirth of the sun, but by the 16th century AD it was falling around December 11. This had happened because in reality a year lasts slightly less than 365¼ days and so leap years need to occur slightly less often than once every four years in order for the solstice to remain at a constant point in the calendar. Pope Gregory XIII, in his capacity as Pontifex Maximus (which the popes had inherited from the Roman emperors after the fall of the Roman Empire), instituted a change, and this revised calendar is known as the Gregorian Calendar. He removed ten days from the year 1582, bringing the winter solstice to December 21 (not quite where it was in Caesar's time), and then decreed that leap years should be dropped from every year ending in double zero, unless it was also divisible by 400. So, 1600 and 2000 were leap years, but 1700, 1800, and 1900 were not.
In 1582, however, the pope's writ no longer ran all over Europe, because of the Reformation. Protestant countries were very slow to adopt this new papal calendar, and Great Britain and its colonies did not adopt the new calendar until 1752, by which time the difference between the two calendars had risen to eleven days. The Act of Parliament that instituted the new calendar stipulated that all birthdays should be expressed on the new calendar. This is why, for example, that George Washington's birthday is celebrated on February 22, when he was actually born on February 11  at a time when the old calendar was still in force.