It is generally best to do your pruning in the spring.  For spring-blooming bushes such as forsythia, lilac and spirea, prune after they’re done blooming.  Summer blooming plants such as rose-of sharon and butterfly bush can be  pruned in the summer after they bloom.

Nothing should ever be pruned in the fall.  Pruning at this time only stimulates growth when the plant is going dormant.  New growth will be quickly killed by frost.