Re: Leap Weak Calendar ISO rule
            Wed, 12 Mar 1997 10:10:40 -0800
            Simon Cassidy <>
On Wed, 12 Mar 1997 Lars Osterdahl  wrote:
> The 'first' week of a year is the week that has more than 3 days
> (starting counting week days with monday, as the European standard says).

On Wed, 12 Mar 1997 13:05:06 IST Amos Shapir wrote:
> This boils down to a simple thumb-rule for the week containing January 1:
> "the year that has Thursday has the week"....
> Another useful rule: "a year has 53 weeks iff it starts or ends on a
> Thursday". The beauty of it is that it applies both to regular and leap
> years!

Simon notes:

Amos' "Thursday rule" for ISO "leap-weeks" is equivalent to the following:
A calendar year has 53 ISO weeks if, and only if, the year has a dominical
letter "d".

Common years with the single dominical letter "d" begin and end on a Thursday.
Leap years (which have 2 dominical letters) that begin on Thursday, have
the dominical letter "d" until March, and then the dominical letter "c".
Leap years ending on Thursday, have dominical letters "e" and then "d".

This "Thursday rule" or "d-letter rule" is true for the ISO week-
-count whether or not a year has a February 29th. (as Amos points out), but
it is also true whatever rule is used for determining whether a year is leap
(i.e. has a February 29th.).

Thus, an official reform of the Gregorian leap-year rule, (for instance to the
Dee-Khayyam 33-year rule) would not affect the ISO leap-week "Thursday rule".
In order to unseat the ISO "d-letter/Thursday rule" you would need to have
year-lengths other than 365 or 366 days.

Dee's Y'rs, Simon Cassidy, 1053 47th.St. Emeryville Ca.94608. ph.510-547-0684.