Friday the 13th

[nq:1]Friday the 13th came on Friday this month (as Pogo would say). Is the probability of that happening exactly one seventh, or is it more complicated than that for some reason? Comments?[/nq]
The probability of Friday the 13th being on a friday is 100%.

[nq:1]Friday the 13th came on Friday this month (as Pogo would say). Is the probability of that happening exactly one seventh, or is it more complicated than that for some reason?[/nq]
It is more complicated than that because different months have different numbers of days. You will find Friday the 13th occurs twice in some years and at other frequencies in other years, varying according to th

[nq:2]Friday the 13th came on Friday this month (as Pogo would say).[/nq]
Actually, it was Churchy (Cherchez LaFemme) who was obessesed with Friday the 13th, irrespective of what day it actually fell on.
[nq:2]Is the probability of that happening exactly one seventh, or is it more complicated than that for some reason? Comments?[/nq]
[nq:1]The probability of Friday the 13th being on a

[nq:1]Friday the 13th came on Friday this month (as Pogo would say). Is the probability of that happening exactly one seventh, or is it more complicated than that for some reason? Comments?[/nq]
Strictly speaking, the probability of Friday the 13th falling on a Friday is 1.0, but to say in Pogo-talk "Friday the 13th came on a Friday" means the same as most people saying "The 13th fell on a Fri

[nq:2]Friday the 13th came on Friday this month (as Pogo ... or is it more complicated than that for some reason?[/nq]
[nq:1]The probability of Friday the 13th being on a friday is 100%.[/nq]
Not a Pogo reader, eh?
Brian Rodenborn

Raymond O'Hara:
[nq:2]The probability of Friday the 13th being on a friday is 100%.[/nq]
Barbara Need:
[nq:1]... I suspect the the OP meant what are the odds of the 13th of any month falling on Friday.[/nq]
If the number of days in one cycle of the leap-year pattern under the Gregorian calendar wasn't a multiple of 7, then the probability would necessarily be exactly 1/7, but it is

[nq:1]Friday the 13th came on Friday this month (as Pogo would say). Is the probability of that happening exactly one seventh, or is it more complicated than that for some reason?[/nq]
The modern calendar repeats every 400 years. In that time there are exactly
12 x 400 = 4800 months, each with one 13th day. If you count all of themthat land on a Friday, you should get 688. That sets the pr

[nq:1]Actually, it was Churchy (Cherchez LaFemme) who was obessesed with Friday the 13th, irrespective of what day it actually fell on.[/nq]
Nobody seems to remember that but you and me. Every once in a while I used to make a remark like, "Friday the 13th comes on a Tuesday this month," and all I would get back were blank stares so I stopped doing it.

John Varela
(Trade "OLD" lamp

[nq:2]Actually, it was Churchy (Cherchez LaFemme) who was obessesed with Friday the 13th, irrespective of what day it actually fell on.[/nq]
[nq:1]Nobody seems to remember that but you and me. Every once in a while I used to make a remark like, "Friday the 13th comes on a Tuesday this month," and all I would get back were blank stares so I stopped doing it.[/nq]
Me too, and I remember it w

[nq:1]Over the 4,800 months in the cycle, a simple brute-force check (using "cal" on UNIX) shows that the 13th falls: ... Friday 684 times on Saturday So the probability is 688/4800 = 43/300, or just slightly greater than 1/7 = 43/301.[/nq]
And from that we can see that the most likely day for the 13th to fall on is Friday. Figures.

Evan Kirshenbaum + HP Laboratories >Active