spring apple tree prunning
You can prune an apple tree any time of the year without hurting it, but late winter, just before spring, is probably best. The worst of the cold weather is past, so you won't be subjecting the fresh cuts to severe icing, but you'll still be able to influence the tree's spring growth.
The following publication has tips on pruning: http://extension.oregonstate.edu/linn/sites/default/files/pruning_pub_handout2012.pdf
Thanks for using Ask an Expert. Join us at the Master Gardeners' Spring Fair on May 6 and 7 at the Expo in Central Point.