This question depends on where you live.  You do not want to spray for weeds too early or too late in the year.  However, we are a Minnesota based company and October is not too late to spray for weeds in MN.  Between mid-September and late October, it is one of the best times to spray for some weeds, especially perennial broadleaf weeds.  When the weather gets cooler, the weeds move nutrients to their roots which means the treatments you spray will be absorbed better and be more effective.