Doing it in odd months makes it coincide with International Mafia Month, which is the source of the original Marathon Day.
Third weekend should work fine without running into major holidays. That would be:
- September 16-19, 2011
- November 18-21, 2011
- January 20-23, 2012
- March 16-19, 2012
- May 18-21, 2012
- July 20-23, 2012