When is the next "double-13 Friday"?

[Joshua Zucker]

On 10/14/06, Jon Wild <wild at music.mcgill.ca> wrote:
> On Sat, 14 Oct 2006, Jonathan Post wrote:
> >> Conjecture: The same basic repeating pattern results
> >> if we seek the number of "Sunday the 22nds" or
> >> "Wednesday the 8ths" or anything else similar, with
> >> the only difference being that the sequence starts at
> >> a different point in the repeating pattern.
>
> I don't think your conjecture is correct, because I remember working out
> once that the 13th falls more often on a Friday than on any other day of
> the week. (This is possible because the whole 400-year cycle of the
> calendar contains a number of days that is not a multiple of 7.) So the
> number of Thursday-the-13ths, Wednesday-the-13ths, etc., per year cannot
> follow the same pattern as the number of Friday-the-13ths per year.

I think everything Jon Wild writes above is correct except for the
word "not" just preceding the "a multiple of 7".  That is, the 13th
falls more often on a Friday because the whole 400 year cycle of the
calendar contains a number of days that IS a multiple of 7.

--Joshua Zucker