There are some who breed their sheep much later in the year or even in early Spring. A few breeds will come into heat then, but it isn't very common. A lot of lamb meat gets frozen or imported from Australia, which are on the opposite seasons from us.
Hair breeds come into heat and breed well in the fall as well as the spring...pretty much any time, really, so very easy to have spring market lambs from hair sheep. More folks in the US are moving towards hair breeds for the meat market and we even have a new hair breed called Royal Whites that has been developed recently from Dorper/St. Croix mix.