## 13 July 2007

### roller-coaster of a week: Friday the 13th

Just six days after "the luckiest day of the century", we have Friday the 13th, supposedly the unluckiest day.

The 13th day of a month occurs on Friday more often than on any other day. But it's not that often -- 688 times every 400 years. You'd expect 4800/7 = 685.714... times.

Perhaps there should be a tradition of, say, Wednesday the 25th being a day of good luck. (Let's see -- maybe Saint Nicholas was born on a Wednesday, and the 25th of December is Christmas...) Of course Friday the 13ths and Wednesday the 25ths would always occur in the same month. I wonder how long it would take the average person to realize that the good-luck-day and the bad-luck-day always happening in the same month was more than a coincidence. The most likely day for Christmas, by the way, is not Wednesday -- it's a tie between Sunday, Tuesday, and Friday.

I'd heard that the 13th occurred most commonly on a Friday before. A few weeks ago I tried to compute the probabilities in my head. Unfortunately they are all so close to one-seventh that I couldn't, at least not while walking.

To calculate days of the week in general, in your head, you can use the Doomsday Algorithm, created by J. H. Conway. I seem to recall reading once that Conway had his computer programmed so that he couldn't log onto it unless he could determine the day of the week corresponding to a given date within some (short) amount of time; this may or may not be true.

Last but not least, the Phillies. I have tickets tonight. They will lose, although I don't want them to, and that will be their 10,000th loss. How do I know this? Because they're playing the Cardinals. And game four of the 2004 World Series -- the one the Red Sox won in four game -- was played against the Cardinals, during a total lunar eclipse. Historic things happen when the Cardinals are playing and there's already some sort of bad omen going on.

(Note to the stupid: I don't really believe the previous paragraph.)