Pruning trees and shrubs can be done almost any time of the year. I find it easier to do in the early spring just as the plants are waking up from their long winters nap. I can see the new grow and the I can better identify the dead branches. Flowering trees and shrubs are best pruned just after they flower. Evergreen trees and shrubs are a little trickier to prune than the deciduous ones. I am a horticulturist by education and by heritage. The books taught me many things but my grandparents taught me things that one will never find in the classroom.
One of those lessons they taught me was that no matter I am pruning or what time of year it is, pruning should always be done on a cloudy day. The cutting of branches is quite shocking to the plant and if it is done on a hot sunny day the plant is forced to heal at an accelerated rate. It some cases the plant can not do this and will go into stress and possibly die. The cloudy day says to the plant that it fine to slow down and time its time to heal from the pruning. It also allow the plant to taking water slowly and avoid being stressed. Stressed plants invite insects and disease.
In the early spring, I clean my pruners very good with bleach to kill any latten disease from earlier prunings. I oil them very well and sharpen the blades. As the new buds start to peak out I carefully prune the trees and shrubs one branch at a time. This is called selective pruning and it is much healthier than electric pruning shears. By selectively pruning you can the direct the new grow in the direction you want it to go giving you trees and shrubs and much better chance to develop evenly and properly.
Flowering tress and shrubs should be given the chance to flower and show their luster before pruning. But just as they have done their thing it is time to selectively prune . The plant will immediately start to form the buds that will become next years flowers so you want to shape the plant before you are in danger of stealing any of next year performance. If you prune in time the plant will form the future flower buds in an even fashion all over.
Evergreens are much harder to prune than any other. They do not have the ability to grow the terminal buds needed for regrowth from the pruning point. You have to be a real visionary to properly prune an evergreen as the new growth happens from two points at the pruning site. By this I mean the plant will respond with two different brand new branches unlike the deciduous plants that respond with one dominant branch. I personally do not prune an evergreen unless it is to remove dead growth. I let the shape develop as nature intended. When you buy an evergreen, pay close attention to the size it will become and make sure that is in fact what you want.