Anytime now through till spring, to be honest, you could prune it anytime and it will just keep growing, you will never damage it by pruning, Except spoil the shape maybe, prune it hard, they grow really fast.
Hi annie tree, The above advice is good but Persoally I would prune weeping willow in the late autumn or winter but no later than January, as its sap flow rises very early and is one of the first trees to start greening up.
If you hang on till after the leaves have fallen and look at it against the sky, you may get a better notion of where to start pruning. If it 'bleeds' sap heavily it may become prone to fungal attack. Good Luck Tbird+