So, you may have heard a rumor on Twitter, or by email, or wherever it is that rumors get started, that last October (2010) had 5 Fridays, Saturdays, and Sundays, and that this happens only once every 823 years. It’s not true!
This silly piece of nonsense circulates via email and social networking websites and is now in its third incarnation. The message imparts the "interesting" fact that July 2011 will have 5 Fridays, 5 Saturdays and 5 Sundays. It claims that such a combination of days only occurs once every 823 years. It also claims that those who forward the message to their friends will receive money within four days.
It is perfectly true that July 2011 will have 5 Fridays, 5 Saturdays and 5 Sundays. However, the claim that such an occurrence for July only happens once every 823 years is nonsense. In fact such combinations occur in the month of July every few years. As the following calendar shows, the next time a July has 5 Fridays, 5 Saturdays and 5 Sundays will be in the year 2016:
This silly piece of nonsense circulates via email and social networking websites and is now in its third incarnation. The message imparts the "interesting" fact that July 2011 will have 5 Fridays, 5 Saturdays and 5 Sundays. It claims that such a combination of days only occurs once every 823 years. It also claims that those who forward the message to their friends will receive money within four days.
It is perfectly true that July 2011 will have 5 Fridays, 5 Saturdays and 5 Sundays. However, the claim that such an occurrence for July only happens once every 823 years is nonsense. In fact such combinations occur in the month of July every few years. As the following calendar shows, the next time a July has 5 Fridays, 5 Saturdays and 5 Sundays will be in the year 2016:
And the same combination of days occurred in July 2005:
And, in any case, there is nothing even remotely unusual about months that have such "interesting" combinations of days. In fact, any month that has 31 days will have three consecutive days that occur five times in the month. Such combinations are commonplace and occur each and every year.
The message is a revamped version of similar - and equally nonsensical - chain letters about August 2010and October 2010.
Let’s think about this, a year can only start on one of seven days, so there are seven possible basic calendar years. Add leap years, and there are fourteen basic calendars. Period. And one of those calendars only gets used every 823 years? How would that be possible? It’s not of course, all fourteen calenders get cycled through regularly, in fact 2010 uses the exact same calendar as 1999.
Here’s the 2010 October calendar
And here are the calendars for October 1982, October 1993, October 1999, and October 2021. See a pattern?
To save you the trouble, 1971 and 2004 had the same October calendar, and 2032 will have the same as well. Hardly a once-every-823-years event.
(PS. There’s a pretty simple explanation for this all: For a 31-day month to have 5 Fridays, Saturdays, and Sundays, the first day needs to be a Friday. Each nonleap year has 365 days, which is 52 weeks plus one day [52*7=364; 364+1=365.] So every passing year will ‘push’ the first day of the month forward by one day, defining ‘forward’ as Friday -> Saturday, Saturday -> Sunday, etc. However, if it’s a leap year, the first day of the month will be pushed forward by two days. Since any six-year cycle will contain at least one leap year, this means that the same October calendar should reappear every six years [five one-day pushes forward and one two-day push = one full week covered] unless there are two leap years that fall in that space, in which case the whole cycle will be shifted forward by one day.)
