Ok, I figured it out, pretty simple really. In addition to a weekend-only calendar, we have a recurring meeting every Thursday, and at some point in the project I have to make a presentation at that meeting, so I need to force the even to fall on a Thursday. Here's how I did it ...

I created a new calendar, and created an exception, called it "thursday meeting".
Then I clicked on the "detail" tab on the right side of the window, then selected "recurrence pattern - weekly" and checked every day except Thursday and marked them as "non-working".
Then I went to the "range of recurrence" and selected "end after" and set the value to 999.

That's not as long as you might think, because every day is considered an occurence, so it actually end in about 166 weeks ... 999/6.