Something like this should work for you -
DateTime GetDateFromDay(DayOfWeek dayOfWeek, int searchDirection)
{
if (Math.Abs(searchDirection) != 1)
throw new ArgumentException("Must be 1 or -1", "searchDirection");
DateTime testDate = DateTime.Today;
while (testDate.DayOfWeek != dayOfWeek)
testDate = testDate.AddDays(searchDirection);
return testDate;
}