Tuesday, 5 October 2010

Once every 823 years?

One of my favourite topics of rants in the real world is how gullible the internet is making us all.

Until fairly recently, we'd not have believed what people say to us unless they had some kind of reputation for telling the truth and/or knowing a lot about the subject upon which they are expounding.

Yet nowadays millions of people repeat "stuff" as if it was a fact with all the authority of a national academy of sciences, just because they saw it online. 

I've been especially amazed over the last few days by a statement that keeps coming into my email inbox and has taken Twitter by storm:

The basic form  is:

This month has five Fridays, five Saturdays, and five Sundays, and this only happens once every 823 years!

Well yes, October 2010 does indeed have five "long weekends". But is this really such a rare occurrence?

Think about it for a moment:

In order for a month to have five Fridays, Saturdays and Sundays, it must be a 31-day month (January, March, May, July, August, October, December)  and the first day of that month must be a Friday (so that it's Friday, Saturday and Sunday which are repeated rather than any other contiguous 3 days).

A 365-day year is 52 weeks and one day. As a result, successive years start on the day of the week after the year before (eg 1st October 2009 was on a Thursday, 1st October 2010 was a Friday and 1st October 2011 will be a Saturday). So in principle every seven years should be the same.

But we get leap years every 4 years (minus a few corrections) so the changes aren't all that regular. But on average every eight years we get the same days of the month on the same days of the week - nowhere CLOSE to 823 years!

Just so that you don't have to take my word for it, check any calendar (there are loads online), for how many Fridays, Saturdays and Sundays there were in October 2004 and 2010, and will be in 2021.

The basic numbers of days and months in a year (a 52 week year equals 13 4-week "months" rather 12 variable-length months) mean that, on average, every EIGHTH MONTH has five Fridays, Saturdays and Sundays which makes the event in general actually quite common. May 2009 and January 2010 were recent examples, and the next one after October 2010 is July 2011. Ask anyone who is paid every fourth Friday rather than calendar monthly for their experience.

Furthermore, the WHOLE of the 2010 calendar is exactly the same as 1999.

What has happened to the world and everyone's faculty for critical thinking if they accept this idiotic 823 years statement without a moment's hesitation?

Is it any wonder that the creationist/young earth movement is getting more and more adherents if people accept what's said to them without thinking about it?


I'm especially concerned by a variation on this meme which adds that years with a date which can written 10/10/10 only happen every 823 years.

How the FUCK can a decimal year (i.e. a year ending in 10) happen with a frequency other than 10 years apart? 10/10/10 happens every 100 years, the next time being in 2110. 

I can't be bothered to do the exact calculation, but the combination of 10/10/10 AND the five long weekends in October happens roughly every 400 years.

In any event, it's NOT going to be a frequency ending on 3!

823 years from now is going to be 10/10/2833

Surely nobody even has to THINK about this for more than a millisecond? Surely it is self-fucking-evident?

Why the hell do people think that retweeting/re-emailing this kind of complete BOLLOCKS makes them appear to be anything other than illiterate, innumerate and incapable of rational thought?

Comments welcome, because I am really curious why being given a network connection seems to make otherwise intelligent people into gibbering baboons?


  1. I Thank you! lol...I just pissed off my 15 year old who came to me with the "big News" that Oct 2010 has 5 weekends and it only happens every 823 year's story.
    When I kindly told her,that NO,that is not entirely the truth and why...she stomped her foot and said 'Geez-us MOM,how come you always have to prove me wrong! I was just trying to share an interesting "fact" with you"...to which I replied...It is not a "FACT", didn't you listen to anything I just said?
    Her reply was,...No it's a Fact I just read it on FB! After banging my head on the desk I wrote this and posted your Blogspot on this subject to her FB wall : )

  2. I saw someone on Facebook with the status of 5 weekends in October only happens every 823 years. This didn't make sense to me what so ever so I used the power of the internet to find out the truth behind this. The internet took me here. Thanks for confirming with me that this is a untrue. People are WAY to gullible!

  3. What about July 2011 ? If I'm not wrong there are 5 long week-ends again (friday/saturday/sunday) ?


  4. wingman, that issue is addressed in the blog.

    Some people claim it's just October having 5 long weekends that occurs every 823 years, some claim it's any month. The blog deals with both, and July 2011 is already mentioned.

  5. The "long weekends" (5 friday/saturday/sunday) happens every 11 years. As long as the 1st of the month falls on a Friday, 11 years from then, it will happen again....

  6. I like it. However...

    How do you work out that a date which can be written as 10/10/10 occurs "roughly every 400 years"? Surely it's once a century? Assuming the dd/mm/yy format, you've got 10/10/10 in 2010, then 100 years later you've got 10/10/10 in 2110, and you've got it again in 2210, and so on.. every 100 years.


    To be honest, I hadn't heard the five Fridays, five Saturdays, five Sundays meme, but pretty much every year since 2001, some chump has exclaimed (either in an email or by some status update) that, "oooh, tomorrow the date and time will be 01/01/01 01:01:01 and this only happens once very 823 years/once in a lifetime/once EVER" or something along those lines. Which is what lead me here this evening, seeing as tomorrow’s 10/10/10 10:10:10 and everyone’s getting very excited about it on Facebook.

    If you think about it for - to use your phrase - ‘more than a millisecond’, you realise that this particular phenomenon happens every year for 12 solid years at the start of each century, from the year '01 through to '12.

    Your guess is as good as mine as to why people lap it up so readily.

    Anyway, nice blog.

  7. Thanks for your thoughts, DarkC.

    What I meant by "approximately every 400 years" was the COMBINATION of 10/10/10 AND the 5 "long weekend". Of course, 10/10/10 happens every century, but as far as I can tell, the five Fridays, Saturdays and Sundays happen a year or two away.

    I THINK the next 10/10/10 in a five long-weekend October will be in 2410.

  8. hahah i immediately recognized someones facebook status as this as bullshit. and it made me mad to see people hitting the like button without scrutinizing the certainty of what someone is claiming to pass off as a fact. when i read that fb status i immediately disproved it by looking at my desktop calendar, which u can easily scroll back years and see how the days are arranged and 2004 was the last occurence of october having 5 fridays, saturdays and sundays. great post. ill have to check on when the next coincidental event of it being 10/10/10 and having 5 fridays, saturdays and sundays will be.

  9. Okay yes I have to admit that I didn't actually think about it before retweeting that "fact", but in my defence, I am seriously crap with numbers and therefore it's one of the few areas in which I will accept whatever I'm told without question because I don't have the mental capacity to verify it myself! ;) Not much of a defence really, but other stuff I would generally try and check out first :)

  10. Not trying to hate, but, um, zanyzigzag, you say you are "crap with numbers". In your "defence" I think you might be crap with spelling also.

  11. check out october 2021 and slap the person who told you this FACTOID

  12. Dark C is quite right. Dates like 10/10/10 (or similar, e.g.01/01/01 . . . 05/05/05 up to and including 12/12/12) occur every 100 years, not every 400 years, assuming one is allowed to exclude the "century" (20,21,22 etc) from the date format.

    Plum reminds us that every 4th year is a Leap Year. How many people out of the next, say 100 that you talk to can explain WHY we need Leap Years in the first place? (I once gave a talk on IT to 200 undergraduate engineers in a prominent UK University and NONE of them could . . . ) . . . and what would we observe if we didn't have Leap Years.

  13. Plum . . . any paticular day falling on a particular date happens every 11 years, except at years xx00, which are not always Leap Years.
    (HeHe . . .why not?)
    So, as you say, the next time 1st. October is a Friday is in 2021, then 2032 etc.
    The easy way to check this sort of thing is to use an app such as Macintosh's vastly underrated "Hypercard" whose "long date" function will give you the day of the week for any Input date.

  14. Here's another piece of the sort of factoid bullshit that appears on Facebook, Twitter, Blogland, Colleges of Further Education etc.

    "There must be MILLIONS of 4 digit PIN Numbers because the Banks tell us our PIN number is unique to us, and the banks have millions of customers." Q.E.D.

    O.K., Facebook factoid fanatcis and other bullshitters . . how many 4 digit PIN numbers are there, assuming all combinations are allowed?