Scheduled workflow that checks dates of tasks

You can likely use DateAddCalendarFixedPeriod, then compare. 

• Let dueDate = DateAddCalendarFixedPeriod(Today(), "w", 6)
• If $StartDate = dueDate, do something

Though, I will say, Schedule Workflow Rules, in Clarizen, are highly unreliable. The queue/scheduler doesn't work the way you think it works. Likely, to reduce performance impact, items will only appear on the scheduler if they've been "touched" within given period.