This is definitely not a simple question and research can be conflicting.  In many parts of the world pruning is done throughout the year. When making a decision regarding the care of valuable trees on your property, make sure to always consult  a professional tree care company with an ISA certified arborist.

New research is starting to demonstrate that contrary to old methodology of pruning during tree dormancy, it may be better to do pruning on live branches in the late spring and early summer following a flush of growth after leaves harden and turn dark green. Research in Europe is suggesting that less stem decay occurs when pruning in the summer as compared to dormant season pruning.

However, during the initial flush of growth in the early spring trees have allocated a lot of energy to growth and may have depleted energy stores.  This is especially true for trees that are already stressed. Bark and Cambium are easily damaged and separated from wood when pruning takes place during a growth flush during this spring growth because early xylem is gelatinous and slippery.

*Avoid pruning at times of low energy reserves, when a tree is stressed, and when soil is uncharacteristically dry or flooded. (An Illustrated Guide To Pruning-Gilman)

It seems that pruning during this time allows trees to seal wounds from the drying effects of air better due to active defense substances that depend on cell growth.  Parasites have a hard time colonizing fresh-wounds when these defense compounds are active.  Woundwood and callus wood are also formed during this time further slowing decay. An exception to this rule might be in desert climates where trees have low moisture content in the summer.  This might also apply to late-summer or early-fall pruning where wood water content is low and disease causing fungal spore counts are high. This spore count remains high in warmer areas of the world even in the winter so the popularity of pruning during this time to reduce fungal infection may not be effective.  This principle applies at anytime of the year if the area is experienced severe drought conditions.

Heavy pruning in the late summer or early fall can initiate a late season flush of growth and delay dormancy.  This new growth has not been hardened off to the cold and can be damaged by early frost.  In addition the cambium layer around pruning cuts at this time of year is more susceptible to dieback and the pruning wounds are more prone to cracking.

When heavy pruning is done in the late winter just before the spring growth flush, the tree is defense systems are activating but the large associated energy expenditure has not happened making it a good time.

As far as growth control, non-pruned trees grow the fastest.  By removing live branches you are removing photosynthetic capability and energy reserves causing a dwarfing effect.  If your pruning objectives are mostly pertaining to growth control, pruning trees just before the spring growth flush will encourage growth while pruning after this growth flush will slow growth.

“Only healthy, vigorous, young, or medium-aged trees should be pruned using this strategy. Pruning live branches from unhealthy old trees, including those impacted by construction activities, at a time of low energy reserves, during or just after the growth flush, could deplete them further of much-need energy reserves and energy producing tissues” (An Illustrated Guide To Pruning-Gilman)

Light pruning (removing less then about 10% of the foliage on medium-aged trees or 20% of the foliage of young trees) can be done most of the year as long as the branches being cut are small diameter.

Pruning in the dormant season is recommended on certain trees susceptible to diseases and insects (such as borers) in certain areas of the country. During the dormant season there is an overall reduction of inoculum.  Avoiding pruning in wet conditions can lessen the change of fungal infection by reducing the vulnerability of freshly cut wood.  Pruning during times when insect vector numbers are not as active is advised but varies by climate.  For example, an insect vector may still be active in Georgia in November but in Wisconsin the insect vector and fungal pathogen is inactive.  If pruning a tree that is already infected by a disease such as fire blight, avoiding wet conditions and pruning during the dormant season can reduce the spread of disease to other plant parts and other plants.  In the growing season, pruning can open wounds that can start other infections.  Avoid wet and warm weather when pruning infected trees.  Pruning often by making small cuts is also a way to reduce the chances of infection.

Most fruiting trees are best pruned in the late winter just before bud break.  This reduces the chances of winter related damage. However, Pears, apricots, and peach trees among other trees that are susceptible to bacterial canker should be pruned in late fall to avoid infection through pruning cuts.

For the desired multi-stemmed form on young ornamental trees, is is best if they are left un-pruned for 2-3 years to allow for rapid growth.  Pruning lower limbs directly after planting can slow growth.  If the tree is severely headed, the new shoots will be vigorous yet the overall growth of the tree will be slowed.  To avoid having to head these trees to induce a fullness of crown, wait the recommended 2-3 years before pruning.  After the third year, they can be pruned to their desired shape.

In conclusion, pruning recommendations have changed due to ongoing research, which includes conflicting research. This makes sense due to the fact that arborists deal with so many species of trees with different susceptibilities in different climates and environmental conditions. Arborists also work with trees of varying health and vitality.  An arborist must use his expertise, and understanding of these factors while trying to meet their client’s expectations.  This involves making the decision of whether or not the client’s needs can be met while maintaining a healthy tree.