unsigned char Month = 6;         // 1..12
unsigned char Day = 18;          // 1..31
unsigned char Year = 08;         // 0..99 (2000-2099)

//  71 instructions on 12F683 (BootsC 16)

unsigned char DayOfWeek()        // Mike McLaren, K8LH, Aug '07
{                                // returns 0..6 (Sun..Sat)
  unsigned char Leap = 0;
  const char Base [] = { 5, 1, 1, 4, 6, 2, 4, 0, 3, 5, 1, 3 };

  Leap = (Year & 3) || (Month > 2);
  return (Base[Month-1] + Day + Leap + Year + Year/4) % 7;
}