The mop head hydrangeas, also called big leaf hydrangea or Hydrangea macrophylla flower on old wood so for the most blooms they should be pruned in mid summer when you can harvest the flowers and shorten the branches at the same time. Then there is plenty of time for new growth before winter. In cold winter climates the branches that already flowered will not bloom again for two years so you can cut back the faded brown blooms from last year now (in winter) but do not shorten the branches with no old blooms as they have flower buds at the tips waiting for this coming summer to bloom. Keep in mind that most hydrangeas bloom best with no pruning at all. Just let them grow into huge flowering shrubs. If you have a very mature hydrangea you can renew it by cutting back the thickest and oldest branches by one third. I have also had good luck renovating my huge hydrangeas by cutting the thickest old branches all the way to the ground in winter or early spring. Also remove anything that is dead, diseased or damaged. I have a hydrangea room in my own garden and I post photos of this on Facebook and Instagram and have also done videos for PlantersPlace on Youtube showing you how I prune my own hydrangeas. Thanks for the great question and Keep Growing, Marianne Binetti