Pruning of deciduous trees (those that lose their leaves in winter) is often best carried out in winter, as it is easier to see the branch structure.
Pruning of evergreen trees is often carried out in April, as they come into growth.
Exceptions to these rules include tender deciduous trees, best pruned in spring, once the risk of frost has passed, and also stone fruit trees (cherries, flowering cherries, apricots, peaches, plums and nectarines). These can be pruned in winter while young, but are later best pruned in summer. They can be at risk of catching the diseases silver leaf and bacterial canker if large branches, those thicker than your wrist, are pruned in autumn or winter.