The origin of the Calendar is tough to pin down. We know that many ancient peoples had their own calendars.
Many primitive cultures used a four-day week, possibly in honour of the four directions. Central American peoples used a five day week; Assyrians (an ancient people who lived in what is now Turkey) had a six day week; pre-Christian Romans in the 1st century BC had weeks of eight days called nundinae.
For many centuries, ancient Greeks - like Babylonians (ancient people who also lived near what is now Turkey and Iran) and Egyptians of the same period - divided their 30-day months into three “decades” of ten days. (Egyptians called their ten-day period decans.)
The biggest change in calendars was brought about by Julius Caesar in 46 BC. Caesar, taking advice from an astronomer named Sosigenes, extended that year (46 BC) to a total of between 443 and 445 days. He then extended future calendars to 365 days a year. The 365 day calendar is called the Julian Calendar in his honour.
Julius Caesar also added an extra day in February (making a “leap year”) every four years, without exception. As it turned out, this was too much – it added almost eight more days than necessary in each thousand years. As the years passed, the calendar became further and further out of phase with the seasons. As time went on, the summer season started in October!
In 1582, Pope Gregory XIII did two things to correct this problem. First, he shortened October of 1582 by ten days. Then he ruled that any year whose number ended with 00 must also be evenly divisible by 400 in order to have a 29-day February. Confusing? What Pope Gregory essentially did was re-adjust the calendar so that the seasons would always match up to the months they were supposed to. —Daily Times Monitor