Why is everybody keep using Mon, 26 Jul 1997 as a date in the past (PHP Headers)?

In order to disable browser cache on dynamically generated pages, PHP uses the header method below:

header("Expires:  Date_in _the_past");

Over the years most people have been using the following:

header('Expires: Mon, 26 Jul 1997 05:00:00 GMT');

If you don’t believe just do this Google Search.

There are only a couple of problems however:

FIRST PROBLEM : Jul 26 1997 fell on a Saturday not on a Monday.

SECOND PROBLEM : Why does it have to be that date?

There are plenty more dates to choose from anyway, so why is that one (Mon, 26 Jul 1997 ) or should we say (Sat, 26 Jul 1997) so special?

If you have any ideas, please drop me a comment.