Late winter is the best time to trim. Also, if you have trees that haven't been trimmed your best bet is to take it in stages instead of trimming the tree in one year.Here is a link for info. There is plenty of info on the web as well....just Google it!

http://www.motherearthnews.com/Homesteading_and_Self_Reliance/1976_November_December/It_s_Pruning_Time_for_Apple_Trees