Today I was a bit impulsive and registered the domain name weeknum.org.
Then I made a little php-application showing the day of the week, with the background color calculated using the week number, month and day to create a hex code.
I might even try out some AJAX and jQuery on this page and see if I can make it a little more stylish and interactive.
If you want to check start and end dates of another week number, just add the suffix /[week number] to the URL.
For example, if you want to find information about week number 42, then go to weeknum.org/42.