How to find the 3rd Friday in a month with C#?

Question

Given a date (of type DateTime), how do I find the 3rd Friday in the month of that date?

Answer 1

My reasoning goes like this

• the 15th is the first possible "third Friday" (1,8,15)
• therefore we're looking for the first Friday on or after the 15th
• DayOfWeek is an enumeration starting with 0 for Sunday
• Therefore you have to add an offet of 5-(int)baseDay.DayOfWeek to the 15th
• Except that the above offset can be negative, which we fix by adding 7, then doing modulo 7.

In code:

public static DateTime GetThirdFriday(int year, int month)
{
   DateTime baseDay = new DateTime(year, month, 15);
   int thirdfriday = 15 + ((12 - (int)baseDay.DayOfWeek) % 7);
   return new DateTime(year, month, thirdfriday);
}

Since there are only 7 possible results, you could also do this:

  private readonly static int[] thirdfridays =
      new int[] { 20, 19, 18, 17, 16, 15, 21 };

  public static int GetThirdFriday(int year, int month)
  {
     DateTime baseDay = new DateTime(year, month, 15);
     return thirdfridays[(int)baseDay.DayOfWeek];
  }