Fill weekdays in numbers
It took a little working through, and maybe there is a button I dont know about but...
1) type in start date
2) directly under that cell, insert the following function - assuming that A2 is your cell in step 1
=IF(WEEKDAY(A2,Sunday is 1)=7,(A2)+2, IF(WEEKDAY(A2,Sunday is 1)=6,(A2)+3,(A2)+1))
It looks complicated but it's really just an IF statement with another IF statement embedded in the first if-false.
It drove me crazy trying to find this out so if if still looking for this answer, I'd be happy to explain the steps further.
As much as I love Apple products I think this is one instance where Excel is just plain better than Numbers as far as a spreadsheet goes. And I am not a mathematician by any means.