A few years ago we did a mud bog in Bozeman. I think it was during their Fair though, and if I recall correctly it was Mid-September possibly even into October (if its the one i"m thinking of, they called it Septoberfest, actually). I think there is also a mud bog (but the activites center more on drags, the obstacle and baja) in Cascade somewhere around or on Labor Day... I'll have to ask my hubby.