The ideal time to trim apple trees is late winter, but you can trim into the summer and spring if it's necessary. Avoid pruning the apple trees in the fall because it stimulates new growth while the trees should be preparing for the winter season.
If the apple trees have a lot of new growth, late winter is the perfect time to prune the central leader shoot and other vertical branches that compete the header. Pruning the apple trees is necessary in order to allow light penetration and constant air circulation. This enhances the health of the trees and boosts production of fruits.