One rule covers most of it, and the exceptions are short
The general rule
Prune shrubs just after they finish flowering. That gives a full year to make the wood that carries next year's flowers.
The main exception
Anything flowering on this year's new growth, which is pruned hard in late winter. There are about nine common ones and they are listed.
What not to do
Do not prune spring-flowering shrubs in winter. It is the commonest mistake and it removes every flower bud you were waiting for.
