how to read all the sundays of a month from datetimepicker in c#

I don't think you should try getting "all Sundays" because there are too many. The DateTime class has values between 1-JAN-0001 and 31-DEC-9999.
What you should do is establish a time interval (StartDate <-> EndDate), and store all the Sundays between those two dates.
This code should do the trick:

            DateTime CurrentDate = StartDate;
            //I try to find the very first Sunday after the StartDate
            while (CurrentDate.DayOfWeek != DayOfWeek.Sunday)
                CurrentDate = CurrentDate.AddDays(1);
            //after I found the first Sunday I always skip 7 days (a week)
            //this ensures me that I go from Sunday to Sunday
            for (; CurrentDate <= EndDate; CurrentDate = CurrentDate.AddDays(7))
            {
                //to do: save CurrentDate to the database because it's certainly a Sunday
            }