Now!

Prune after flowering

Immediately after flowering usually.

They will flower next year on the wood they produce this year, so you prune to encourage new growth. Cut off the flowered branches when the flowers die. You have just about time to do it now but don't leave it any longer. A tip with forsythia - cut some sprays at Christmas and bring them into the house and put them on a vase of water in a cool room. Then when you take down your decorations bring them into the warm and they will open a long time before the outdoor ones - spring flowers in the middle of winter!