I suspect, just given the way elections normally work, that it could happen pretty much at any time that engagement takes place. The reason is primarily that sometimes your candidates don't get nominated until into the writ. So I don't think you can specify a time for it. I think, frankly, it just needs to be as fluid as any other assignment form that you would fill in for party purposes.
On June 14th, 2007. See this statement in context.