Some (quite a few) sheep will breed "out of season". No doubt triggered by shorter days ewes will generally breed from September to December fairly reliably. You can use drugs to garuntee a reliable heat cycle and time lambing to the day. The system I am familiar with (and there is at least one other that escapes my memory) uses PMSG to synchronise the ewes, meaning they all cycle at the same time. PMSG is short for Pregnant Mare Serum Gonatropin and is contained in a sponge that is implanted vaginally. After the set time period the sponges are removed and an injection of estrogen is used to actually bring the ewes into heat. Ewes cycle without coming into heat the PMSG just synchronises them. Then with much better than average ram coverage (say 10 /proven mature ram) the ewes will all be bred on that day. Hopefully they do but it does work very well. This system is called "Sponging" in laymans terms. The other system is a feed based if I remember right, but along the same lines.