The best time to do most major pruning is when the plant is dormant (not actively growing). Pruning during dormancy will cause less shock to the plant and it will have a quicker recovery. Dormancy usually occurs in mid to late winter when the temperature is below freezing, but not all plants are dormant at the same time. Plants from subtropical and tropical areas are most dormant during their leafless period and that may not be in the winter. An advantage to pruning in the winter is that without leaves the plants structure is more obvious, making it easier to see what needs to be removed.
There are some important exceptions to the dormant rule. Spring flowering shrubs bloom on stems formed the previous year, if they are pruned in the winter current years buds will be removed and flowering will be greatly decreased. It is best to prune these plants right after they finish blooming. This will encourage them to put out healthy new growth that will bear next years flowers.
Many people wonder if pruning at the wrong time can kill a plant. It depends on several factors including the health of the plant and the climate. It is possible, but not probable, especially if it is only a light pruning. Many plants can tolerate an occasional out of season pruning and healthy plants are better able to bounce back from any sort of pruning than sickly ones. A light pruning is always preferable to heavy pruning when done out of season.
Avoid pruning in the late summer because it promotes new growth that could be damaged if not hardened off before the first frost. It is also important to remove damaged or diseased branches whenever they are first noticed, no matter what time of year it is, to prevent further injury to the plant.