Schedule ad display by day?

I have a site focused on selling a product to professional users, and we’ve noticed that sales (but not page views) plummet on the weekends. I’d like to place display ads on the site, and have them only display on Saturday/Sunday, and weekdays after 8pm.

I thought Ad Inserter had this flexibility, but under scheduling, I’m just seeing very basic “days since publication,” controls. Am I looking in the wrong place or is this not yet a feature?