4 Answers

During winter (when the tree is dormant).  This will invigorate the tree, causing it to grow more during the following season.

When the sap has stopped running....

I prune apples in the winter  and fertilize before spring. When the  trees bud out I go over the tree and wrap wire on excess sprouts at their base to limit growth there and not drain out sap. This method allows me to prune species that cannot be pruned otherwise, such a walnut and fruiting trees. The wire is like coat-hanger wire and constricts a stem or branch from growing. The method allows several interesting potentials that I will be exploring this next season. By reducing sap flow in one branch another should benefit to produce larger earlier fruit. Then releasing the restriction, et cetera. 

Prunes are dried Plumbs, not apples