“Flowers before the end of June, prune soon.” Was a saying that I had heard my Grandfather mutter. And, like many old sayings, there’s truth in it.
Granddad may have been a little flexible on the date as the advice these days seems to be if the shrub (woody-stemmed varieties) flower before mid-June then give it a prune as soon as it has finished flowering. If it flowers after the 16th, then wait to prune it later in the year (or early in the following one).
There are a number of sites listing the types of plants that, in the UK, are likely to need to be pruned at this time and some give details on how to prune. But, I rather liked the comment on one of the BBC pages that “pruning such shrubs is basically deadheading on a big scale!”.
http://www.bbc.co.uk/gardening/basics/techniques/pruning_pruneshrubs1.shtml
http://www.crocus.co.uk/features/_/articleid.978/
http://www.rhs.org.uk/advice/in-month/june/trees-shrubs