A mid-summer prune is safest, to guard against the risk of silverleaf. Autumn to spring pruning, in damp weather, increases the risk of the spores of the silverleaf fungus entering into the pruning wounds, and the fungus, if not caught early, will kill the tree. The fungal spores spread at the greatest rate in damper weather, so pruning is done in summer when the spores are at their lowest level. Pruned wood heals quickly at that time of year, too, lowering further the probability of infection.

My mum always used to do hers as she picked the fruit - the pruning hook was useful for cutting off the higher branches complete with cherries, much to the disgust of the blackbird.