Our system starts weeks on Sundays per the American calendar. In order to avoid splitting a weekend into 2 weeks, you'll need to apply a little trick:
You'll want to enter your time slots for Friday (pretending it's Saturday) and Saturday (pretending it's Sunday), e.g. if you play Saturdays from 9-12 and Sundays from 1-4, you'll enter Fridays 9-12 and Saturdays 1-4.
Once your games have been allocated to those time slots, shift everything back one day: click the first date on the schedule and select the next day. It will ask if you want to shift all games by the same interval (i.e. 1 day). Click yes and you're all set.