how to calculate every monday's date in a date range using start date and number of weeks

Using an arbitary date of Jan 6 1900 as the beginning of time, the formula below 

Math.mod(BeginningOfTimeDate.daysBetween(startDate), 7)

Would return 

• 0 if date__c is saturday
• 1, sunday
• 2, monday
• ...
• 6, friday

As such, below is an apex code that outputs 3 Mondays

Date basis = Date.newInstance(1900,1,6); // beginning of time
Date start = Date.newInstance(2018, 8,1); // start date, input
Integer num = 3; // number of dates to retrieve, input
for(integer i=1; i<=3; i++) {
    //adding days to start date until we get to a Monday
    while(2!=Math.mod(basis.daysBetween(start), 7)) {
        start=start.addDays(1);
    }
    // if Monday, display date then add 7 days
    if(2==Math.mod(basis.daysBetween(start), 7)) {
        system.debug(start);
        start=start.addDays(7);
    }    
}
Hope that helps!