When is it best to prune my Salix Japonica?

Most Salix species are vigorous and grow rapidly, needing plenty of water. The best time to prune is in spring just before the buds start to open.
You can give it a light prune now to shape it, but leave heavy prunning until spring. Prune just above a bud.
Next year after prunning and after the leaves have opened, constantly nip out the elongating growing tips to keep the tree/bush in shape.
I hope this helps, if it doesn't, at least it gave you something to read ;-)

i have a salix standard and was told to cut back where the leaves have gone brown at end of season,guess winter then next year you get new pink leaves, hope this is a help, keeps the shape to