Trees often get forgotten in the winter but there are many advantages to having them pruned in the winter. When the leaves have fallen the structure and direction of the growth of a tree is much easier to see and work on.
Certain diseases such as black knot, are best controlled in winter because the symptomatic branches are much easier to spot and remove. Diseases are also less likely to spread to other trees or through fresh pruning cuts during the dormant period.
Insects are also dormant in the winter which is why Elm trees can only be pruned between October 1st and March 31st in Calgary and all of Alberta. This is to prevent the spread of Dutch Elm Disease which is carried by elm bark beetles that are attracted to fresh cuts and deadwood of Elm trees. If you have a fragile landscape you can take advantage of the frozen ground to protect any potential impact on soft or delicate areas below the tree being pruned or removed.
Summer has so much to offer and so little time to give, so why not take advantage of the fact that you can work on trees and shrubs in the winter and save some of that precious summer time.