The Romans observed the new year in March. Their calendar was continually tampered with by various emperors and became increasingly out of synchronization with the sun. By the year 46 BCE the calendar was hopelessly confused. So Julius Caesar initiated a reform of the entire system and established the Julian calendar which was used in the west until 1582. In the Julian calendar each year contained 12 months and there were an average of 365.25 days in a year. This was achieved by having three years containing 365 days and one year containing 366 days. (In fact the leap years were not correctly inserted until 8 CE).

The discrepancy between the actual length of the year, 365.24219 days, and the adopted length, 365.25 days, may not seem important, but over hundreds of years the difference becomes obvious...the seasons, which depend on the date in the tropical (solar) year, would get progressively out of kilter with the calendar date.

In 1582, Pope Gregory XIII instituted the Gregorian calendar, which has been used since then. The change from the Julian calendar to the Gregorian calendar involved a change of the simple rule for leap-years to a more complex one in which century years should only be leap-years if they were divisible by 400. For example, 1700, 1800 and 1900 are not leap-years whereas 2000 was one. The net effect is to make the adopted average length of the year 365.2425 days. The difference between this and the true length will not have a serious effect for many thousands of years. (The error amounts to about 3 days in 10,000 years.)

The adoption of the Gregorian calendar was made in Catholic countries in 1582 with the elimination of 10 days, October 4th being followed by October 15th. The Gregorian calendar also stipulated that the year should start on January 1st. In non-Catholic countries the change was made later; Britain and her colonies made the change in 1752 when September 2nd was followed by September 14th and New Year's Day was changed from March 25th to January 1st.