Friday the 13th. I have figured out that the most Friday the 13th that can occur in a single year is 3 and the least is 1. What is the most Friday the 13ths that can occur in a 12 month period???

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- October 20th 2008, 09:43 AMbearej50SCARY problem
Friday the 13th. I have figured out that the most Friday the 13th that can occur in a single year is 3 and the least is 1. What is the most Friday the 13ths that can occur in a 12 month period???

- October 20th 2008, 09:47 AMSimonM
Isn't any 12 month period equivalent to some year?

- October 20th 2008, 11:01 AMddt
I assume the OP means by year a "calendar year", i.e. from January through to December. A "12 month period" may also start in, say, April.

ETA:

The reason the answer then may be different is that the weekdays shift one w.r.t. the days of the month in consecutive normal years (and 2 after a leap year).

The way to solve this is to write out for two consecutive years the weekdays of the 13th day of each month, starting with an arbitrary chosen weekday for January 1st of the first year, and see how many identical weekdays you find in any 12-month interval for the 13ths. Of course, you have to do this three times:

1) for two normal years

2) for a normal year followed by a leap year

3) for a leap year followed by a normal year - October 20th 2008, 11:04 AMSimonM
- October 20th 2008, 11:11 AMddt
- October 20th 2008, 12:45 PMLaurent
This doesn't answer your question, but this is a nice fact I think: (It comes from here)

Quote:

In the Mathematical Gazette, vol. 53,, pp.127-129, it is shown that the 13th of the month is more likely to be a Friday than any other day.The author is a 13 year old S.R.Baxter.