See the code below, provided that sDate < fDate.
Add conditions to manipulate the sDate(startDate) and fDate(finishDate)
DateTime fdate = yourFinishDate;
DateTime sdate = yourStartDate;
TimeSpan ts = fdate - sdate;
var sundays = ((ts.TotalDays / 7) + (sdate.DayOfWeek == DayOfWeek.Sunday || fdate.DayOfWeek == DayOfWeek.Sunday || fdate.DayOfWeek > sdate.DayOfWeek ? 1 : 0));
sundays = Math.Round(sundays - .5, MidpointRounding.AwayFromZero);
Btw, code in c#. Good luck!