In the days of the Roman Republic, just prior to its transformation into an empire, Julius Caesar - the great Roman general (and no small shakes as a politician) had a problem and he knew it. No, I'm not talking about his relationship with Cleopatra or with his friend Brutus and the Roman Senate ("You too, Caesar?"), I'm talking about how confounded the Roman system of date keeping had become by the time Caesar became the de facto head of the Republic.
Over the course of Roman history up until Caesar assumed control, the classic calendar in use at the time was subject to manipulation and at the mercy of the whims of various corrupt politicians and other leaders from within the republic. The reason the calendar had become so completely inadequate had its roots in how it was originally set up to keep track of the passing of days and months. It used the phases of the moon rather than the passing of the earth through an orbit around the sun, in other words.
Called the "semi-lunar" calendar, one can recognize that the genesis for the word "month" comes from the word "moon." In fact, our 12 months of the year followed the actual lunar movement, keeping to 29 or 30 days in a month on an alternating basis. Unfortunately, this led to a year of only 354 days, which was fine as far as the moon went, but it came in well behind the 365-day solar year, which our planet adhered to, Roman or other ancient civilization meddling to the contrary.
For this reason, a special month (called an "intercalary" month) was added to the calendar about every 2 to 3 years to get the calendar back on track. Unfortunately, it was a somewhat inelegant way to round out the true solar year and keep the four seasons in their order. Humans being humans, however, a way was soon found to profit from the intercalary month.
Roman politicians and high priests (being the smart people they were) soon came up with a way to use that extra month to put a little coin in their pockets or togas or whatever it was they chose to wear back then. Called "pontifex" or "pontiff," these gentlemen were the deciders of just when the intercalary month was to be added.
Roman consuls (the republic and later empire's chief civil and military magistrates) who happened to enjoy the favor of these pontiffs could expect to enjoy an extra month in office, after payment of a suitable honorarium, of course, through the declaration and imposition of an intercalary month by the Pontiff in office at that time.
Those who didn't enjoy pontifical support would find themselves out of office a month earlier than planned, even though at some points the intercalary month was sorely needed in order to keep the four seasons from going totally out of order. This little bit of corruption had gotten so out of hand by the time Julius Caesar made his return to Rome from Egypt in 45 BCE the Spring Equinox was occurring in real winter.
This, naturally, couldn't be tolerated and so Caesar -- to kick off the great reordering - extended 45 BCE out to a full 445 days, adding the months of "Unidecember" and Duodecember" in order to sort things out. Naturally enough, historians of the day called what went on "the Year of Confusion." It's easy to see why that would've been so.
Next, Caesar took the advice of Cleopatra's astrologer Sosiegenes and revised the Roman semi-lunar calendar to one that fit the Egyptian calendar, called "Thoth." This necessitated a change in the number of days in the month to reflect the new year, which gave the revised calendar 365 days, with an extra day (called, naturally enough, an "intercalary day") added every 4 years.
Because of these changes, 44 BCE and all years thereafter would begin on January 1st, which was the first new moon after the Winter Solstice. Not content with totally reordering the manner in which Rome kept track of the passage of time, Caesar ordered a change to the name of the month of Quintilis (which means "five" in Latin) to Julius, which of course we now know as July.
One might ask why this new intercalary day we now call "leap day" - and the year in which we observe it as "Leap Year" - wasn't put into a day the Romans might have called December 32nd rather than in February, but even Caesar didn't feel he had the power to go against the pontiffs of the day, who'd traditionally observed that time as the point in which intercalary months had been added.
In closing, there is solid historical evidence and scholarly thinking that says Caesar's reordering of the calendar led him to proclaim himself "Dictatus Perpetuus," or "Dictator for Life." It was only one month later - in what is known as the Ides of March - that members of the Roman Senate assassinated him for his temerity in usurping not only the calendar but their own republican ideal of rule.