Here in Texas, I start seeing new Spring growth around March on my hydrangeas. It may be a little later for you, depending on the zone you live in. But for me, it's the perfect time to be Spring pruning hydrangeas. In the past, I tried doing it in the Fall, and every time I did, it stunted the blooms. (insert unhappy face) You want to know why? The blooms are mainly in the new growth, but some varieties will also bloom on old-growth, but I have found that this works best. It can be a real guessing game. Do some research on your variety before trying this method.
Of course, I prune or deadhead old bloom throughout the blooming season, but today I'm strictly talking about Spring pruning.