I dunno about your router (especially as you haven't told me what it is) but my Edimax has a section: Advanced/Scheduling where you can add scheduling rule.
This from the help:
"Schedules can be created for use with enforcing rules. For example, if you want to restrict web access to Mon-Fri from 3pm to 8pm, you would create a schedule selecting Mon, Tue, Wed, Thu, and Fri and enter a Start Time of 3pm and End Time of 8pm."