YahooSarah here. At this time, we do not offer the capability to automatically “daypart” or set ads to only be displayed on particular days of the week. This is certainly a feature we will consider in future releases.
Of course, you can always achieve the same results by turning off your ads or ad groups on Friday evening and turning them back on Monday morning. You can also schedule your campaigns to run in a specific date range by clicking the date range next to “Schedule” on the “Campaigns” page.