Around April, when the new growth is just starting, when you trim off this year's flowerheads and stems, and any other obviously dead bits of wood, trimming back to new growth.

same answer as bamboo gave. diadon

Don't cut off the dead flowerheads as they are protecting the buds for next years' flowers. Wait until all possibilities of frost has passed next year then deadhead, along with pruning.

Prune as sparigly as you can get away with as it will affect the number of flowers you get