When is the next "double-13 Friday"?

[Jon Wild]

On Sat, 14 Oct 2006, Joshua Zucker wrote:

> 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.

D'oh! Thanks Joshua, that's exactly what I meant.