The best season to trim tress is the late winter, just before spring. Wounds left on trees from pruning heal more quickly when spring growth starts, reducing the chances of tree diseases. It is also easier to see the branches and decide where to cut on dormant trees.
If pruning to encourage more flower or foliage growth, do so as soon as the flowers start to die. This encourages the tree to bloom again more fully. If trimming trees because of damage or disease, the time of year is less important than promptly removing branches that are safety hazards or removing dead branches that could ruin the health of the entire tree.