Here's a simple puzzle: In 2012, October 4 falls on a Thursday. On what day does October 15, 2012 fall?

Work it out: October 15 is 15 - 4, or 11 days later. Put another way, October 15 is a week and 4 days later. What day of the week is 4 days after Thursday? Thursday → Friday → Saturday → Sunday → Monday - So October 15, 2012 must be a Monday! If we check, sure enough, October 15, 2012 falls on a Monday.

Now try a similar puzzle: October 4, 1582 also fell on a Thursday. On which day of the week did October 15, 1582 fall? Using the same logic, October 15, 1582 should also fall on a Monday, right? Wrong!

October 15, 1582 was a Friday. How can that be?

October 15, 1582 was not only a Friday, but it was also the day immediately following October 4, 1582. How did that happen? The History channel gives us a brief answer:



October 4, 1582, in Italy Poland, Portugal, and Spain, was the last day they used the old Julian calendar (named for Julius Caesar). As part of the conversion, the calendar was moved forward 11 days, and the new calendar was dubbed the Gregorian calendar, since it was introduced by Pope Gregory XIII.

That explains how the change occurred, but why was the calendar changed in the first place? The answer takes us back to the Council of Trent, at which the Roman Catholic Church worked out how to deal with Martin Luther and his Protestant followers.

Starting at about 6 minutes into the following video, James Burke explains how the Council voted to liven up the churches, but then faced the problem of knowing the right occasion to honor for any given Sunday.



If you watch the following segment up to about the 3:30 mark, you'll learn about Nicholas Copernicus' proposal of a sun-centered system, and the Roman Catholic Church's surprising response.

The reason there are so many problems with finding a workable calendar in the first place is due to quite a few reasons. One of the biggest challenges is that we have to work out our calendar by observing phenomena in out space solely from our perspective on the Earth. Also, our calendar is trying to sync up events that don't really have anything to do with each other. In the following video, C. G. P. Grey does a wonderful job of explaining the challenges of working out the Gregorian calendar:



As I mentioned earlier, the only countries that actually made the change in 1582 were Italy Poland, Portugal, and Spain. Britain and its American colonies didn't make the change until 1752. Ancestry Magazine's article, Time to Take Note: The 1752 Calendar Change covers many of the unusual results the calendar change had, including double dating and the new Quaker dating.

Back in 1982, New Scientist magazine's Making a firm date article covered a larger history of the development of our modern calendar. The information about the problems faced by seasons not properly aligning with the calendar is easy to laugh at now, but it's easy to understand how it could be confusing.