Question

how to find whether the sunday falls between two dates.if sunday exist subtract one days from the difference between two days

Answer 1

If I understand correctly, you are looking for something like this:

public static bool DoesIncludeSunday(DateTime startDate, DateTime endDate)
{
    bool r = false;
    TimeSpan testSpan = new TimeSpan(6, 0, 0, 0);
    TimeSpan actualSpan =endDate - startDate;

    if (actualSpan >= testSpan) { r = true; }
    else
    {
        DateTime checkDate = endDate;
        while (checkDate > startDate)
        {
            r = (checkDate.DayOfWeek == DayOfWeek.Sunday);
            if(r) { break; }
            checkDate = checkDate.AddDays(-1);
        }
    }

    return r;
}

The endDate needs to be the most recent date. The first part simply keeps us from checking if the start and end dates are more than 6 days apart (it will include a sunday, so no need to continue). The second bit just walks backward one day at a time from the endDate checking if Sunday is in there.

Once you know if sunday is part of the span, you can make whatever changes to the dates you want from the calling code.