All you need to do is build a script that resets the counter, then have a cron job execute that script every Monday at 12:00am, or something like that.
If you want it to happen when a visitor comes to your page, you will need to store the date that the counter was reset, then on Monday when a user comes to your page check to see if that date is the same as today, if it is do not reset the counter. If it isn't then reset the counter and set the date to be today.
