The best time to prune shade trees in general is late winter. Pruning oak trees at this time will help avoid problems with oak wilt, to which the trees are more susceptible April-June.
An exception can be made for plants with a high sap flow in late winter, such as maple trees. The best time to prune such trees is late spring or early summer.
Having finished my brief introduction to pruning trees, the final three FAQs of this series will treat three other basic issues in tree care, beginning, in the next FAQ, with fertilizing.