Almost any time is suitable for fruit tree pruning, though apricots and cherries should be pruned only in dry weather during the growing season so the cuts heal quickly and do not become infected.
Summer pruning is recommended to control tree size and because it is a convenient, pleasant time to prune.
Winter pruning is preferred when new growth stimulation is needed more than tree size control, as for less vigorous trees or trees maintained at large size. Also, it is much easier to see limb structure and what needs to be done in winter when the foliage is off the tree.
See also the question "How do I prune my backyard fruit tree?"