Dealing with dates can be tricky. Something along these lines should work:
var number WEEKDAYADD = If(DayOfWeek([Date Two])=6,1,0);
ToDays((FirstDayOfWeek([Date Two])-FirstDayOfWeek([Date One])))/7 + $WEEKDAYADD
Basically, I'm adjusting the dates to both start at the beginning of the week, so that I can calculate the number of whole weeks, between them (and thus the Saturdays). Since the calendar starts on Sunday the first week will always have a Saturday in it. To compensate for the last week I check the day that the date falls on and either add one or zero to the total weeks.
I haven't tested it for all scenarios.
-Sharon