I sure don't know of a way. What do you do now?
All I can think of is to set it for every 9th day, then adjust it to the next Monday whenever it falls on Saturday or Sunday. How often would that happen? ... Well, if starting on a Monday, it would be once a month, because the pattern would be M W F Sunday -> M W F Sunday etc. or T T Sat. M.. hey, a pattern! Very quickly, it always falls on a M W F. So what do you do about M holidays, or other holidays that fall on any other weekday?
Without that problem, you could set three events to on M W F every 5th week. That would work, but would fall on holidays.
P.S. iCal is an Application, not part of OSX.