Plum Tree

I have heard conflicting views on when to prune a PlumTree.  I have an established tree and I need to cut of a bough that has grown over to my neighbour's garden.   Some say I should leave it until mid-summer (July) so that it can heal over the summer and avoid getting silver leaf.   Other's views are that I should do it now - October.   The tree has suffered with the dry weather and has shed most its leaves.     This is a good tree and I do not want to loose it - I would appreciate your views.