Besides the changes of the names of the month, Julius Caesar also enacted
several other important changes to the calendar. The Roman calendar only had
355 days in its year. The solar year has 365.242 days. In attempt to correct
the calendar and keep it in line with the seasons, the Romans would have an
extra month every other year of 22 or 23 days.
The plan worked somewhat, but it actually ended up making the Roman year
longer than the solar year on average by about 1 day. Julius Caesar decided to
end this system, and commissioned a Greek astronomer, Sosigenes, to reform the calendar.
The reform proposed was what we now refer to as leap year. Sosigenes
had determined that the solar year was actually about 365.25 days long. The
length of the months would be changed so that the year lasted 365 days. Every
fourth year an extra day would be added at the end of February, to make up for
the loss of the quarter day between the Roman calendar and the seasons.
Julius Caesar was pleased with this development, and decided to immediately
implement the new calendar. However, there was a problem. The Roman calendar
was already so out of line with the seasons, that they would have to add extra
days to make up for lost time. In 46 BCE, Caesar added two months totaling 67
days between November and December. That year had already seen one of the short
months of 23 days after February, resulting in a year that lasted a total of
445 days.
This year of correction completed, the Julian Calendar went into effect. It
would be the standard calendar in Europe until the 16th
century and the time of Pope Gregory VIII.