These rules took a while to settle down, as I understand it.
For a while different organisations were using different
rules. It seems that the rule is now justified as follows:
Week 1 always contains the first Thursday of January.
Reason - this always guarantees that the majority (4 or more
days) of the first week lies in the right year.
Examples:
The days mon, tues, wed of week 1 may lie
in the previous year, but this doesn't matter for most
office work purposes, because 1 jan is a holiday anyway, and so forth.
The days fri, sat and sun of the last week of
a year may lie in the next year.
But you always get at most 3 days of a week falling in
the "wrong" year.
I.e. the rule is a kind of "error minimization".
The minimum number of days fall in the wrong year, becasue
Thursday is the middle day of a mon-sun week.
Cheers,
Alan Kennington.
