November to Februrary.
I'll assume it's a young tree you have and it's trained against a sunny south facing wall or fance, like peaches and nectarines often are grown this way in the UK.
Prune back the leading shoots to about half the length,always to an outward facing bud, this will cause it to fan-out and keep the middle open and easy to train if you are using wires.
Any Dead, Damaged, Diseased wood or crossing branches should also be removed. Good Luck. Tbird+