Some types of sheep can come into season at any time of year - my bezzie mate used to run an educational farm set-up and had (I think) Dorset Horn sheep in order to maximise the chance of having a ewe-and-lamb pair to show the kiddies. Many farmers breed very early lambs (January, February) to catch the market, but you have to have sheds to do it - I breed a few myself but we lamb late, around the end of April/beginning of May as we can't bring them in. We send the ewes to the ram in November or early December.
Compressing lambing of large flocks into a couple of weeks meant that the farmer can minimise the time that she needs extra hired help.