Hey guys just read this today.
This year, July has 5 Fridays, 5 Saturdays and 5 Sundays. This happens once every 823 years.

Interesting and cool. But, actually, calendars repeat themselves every twenty years.

Or sooner...

http://www.calendardate.com/July/2005_7.php

Actually, calendars only repeat themselves exactly every 400 years, not every 20 years.

For example, March 1st, 2080, falls on a Friday. However, jump 20 years later to March 1st, 2100, and March 1st falls on a Monday!

Barring an unexpected changes in our current calendar format, however, I can guarantee you without looking that 400 years from today, February 9, 2411, will fall on a Wednesday, just as it did this year.

If it weren't for the fact that years ending in "00" are only leap years if they're divisible by 400 (which is also part of the reasoning behind the 400-year rule), the 20-year rule might work 100% of the time, instead of just frequently.



Obviously, if a calendar repeats exactly every 400 years, this isn't true.

Another way of saying that a particular month has 5 Fridays, 5 Saturdays and 5 Sundays, is to say that a 31-day month starts on a Friday. Do you really believe that 31-day months start on Friday only every 823 years?

Guess what? After July 2011, the following months will also have 5 Fridays, 5 Saturdays and 5 Sundays:
March 2013
August 2014
May 2015
January 2016
July 2016

See that? It happens twice in 2016!

Seems like the average of that particular happening is more like once or twice a year.

I've also seen the same e-mail going around using the wording "5 Sundays, 5 Mondays, and 5 Tuesdays", and other assorted day sequences, depending on the month given in the e-mail.

Actually I was wrong with the 20 years. I should have said 28 for the general rule, but I see where you get the 400.

http://askville.amazon.com/years-calenda......15689612

Am still wondering how anyone could possibly have come up with 823.

Good thoughts,

Bob

Now I have some research to do. I hate bad information. It sounded too strange to be made up.

Frequently? Never!

28 year repeats are indeed extremely frequent. See the link I provided above.

Dr. Bob,
Yes, 28-year repeats are extremely frequent.
But 20-year repeats never occur within the Gregorian system.
(You corrected your 20/28 error/typo. I'm just surprised that Scott didn't catch it.)