Shrubs need to be trimmed and pruned primarily to restrict their growth.
The right time to trim shrubs would be when during the mid fall or early spring. Though mid fall would work out better in most cases.
Remember that most shrubs are spring bloomers, pruning them by early spring can disrupt their flowering. If you want them to give a great show during spring, then lay off on too much pruning.
Trim your shrubs as judiciously as possible so that you don't disturb its flowering cycle. If you trim off their flowering branches you will need to wait a good period before the new shoots start flowering. Most shrubs flower on their one year old branches.
Several gardeners prune their shrubs only they are done with the initial spring bloom towards mid summer.
So the two timings which are right for pruning are
While pruning your shrubs annually or during the periodic trim, you should keep the following pointers in mind.
A well trimmed shrub looks elegant and flowers better. Shrubs have a way of growing out of their space within a few months and can cramp their neighbors. If you are using shrubs for your fencing or hedging you would need to be more regular with trimming your shrubs.
Make sure you use sharp tools though and keep your cutting tools clean. Rusted tools can cause infection in plants and blunt tools can crush the branches badly when you trim the shrubs. Happy gardening.