## Sunday, October 10, 2010

### Rare (and not so rare) events and the calendar.

Today is the tenth of October, 2010. No matter if you think of putting the day first or the month first, numerically this is 10/10/10. This doesn't happen very often. Obviously, it happened in 1910 and will happen again is 2110. Also, if you are looking for triple dates like this, they happen in the first twelve years of a century and the last one we will see for a while will be December 12, 2012, unless those silly New Age types who put stock in those pesky Mayans are right and the world comes to an end on the Winter Solstice. That would mean 12/12/12 two years from now will be the last triple date ever.

Don't make book on it. Or more practically, don't buy stuff you can't afford on credit assuming you won't have to pay it off.

So, triple dates. xx/xx/xx only happens twelve times a century. There will also be dates like 6/6/66 or 9/9/99, but those are a little different. So far, so good.

I've also had friends and blog buddies send messages to this general effect.

"This October will have five Fridays, five Saturdays and five Sundays. This happens every 823 years..." or some other equally impressive number.

Impressive, but not even remotely true.

For a month with 31 days, having five weekends in a month like this month is exactly equivalent to saying the first of the month falls on a Friday. Look at the calendar above if this isn't clear. This happens about 1/7 of the time. Because of leap years, Friday, October 1 does not happen every seven years. It happened last in 2004 and will happen again is 2021.

I'd like to say this is just "common sense", but I know plenty of clever people who don't have a reliable numerical sense. Just like an aptitude for music or for draftsmanship, it seems like the most natural thing in the world to people who have it and a daunting task to people who don't. I wrote about this back in August, and I'd like to say I've had a wonderful breakthrough and know exactly how to solve the problem of giving everyone mathematical common sense through education, but that would be a lie. I know if I get all Socratic on a student and ask the right questions, I can get a interested student to see an idea like this with only a tiny amount of prodding. The trick is to get that person to have a little voice inside his or her own head that will ask the right questions.

I have that voice. I can't shut the little bastard up. A lot of people don't have that voice in their heads at all.