It's a Deutzia, probably 'The Magician' although there are one or two
with semi-bicoloured flowers. As for pruning, the best time is
immediately after flowering - do it too late and you lose some or all
of the flowers for the following year. It might be a good idea to
cut half of the biggest stems away now and leave the others until next
year. Although they can be hard-pruned if this is carried out from
the start, some Deutzias react badly to suddenly being cut back hard
after several years of unrestricted growth.