Given a date (of type DateTime), how do I find the 3rd Friday in the month of that date?
I haven't tested this, but since the third Friday can't possibly occur before the 15th of the month, create a new DateTime, then just increment until you get to a Friday.
DateTime thirdFriday= new DateTime(yourDate.Year, yourDate.Month, 15);
while (thirdFriday.DayOfWeek != DayOfWeek.Friday)
{
thirdFriday = thirdFriday.AddDays(1);
}