Is it Presidents' Day or Presidents Day ?

Once one sorts out the confusion between Washington's Birthday and Presidents Day, there is a more fundamental question: How do you spell the non-George Washington version of the holiday ? Many sources differ on where to put the apostrophe in the holiday.

For a lengthy discussion on the issue, see (excuse the title language - it is still interesting):
http://www.blacktable.com/lissner051018.htm