If you're new to sheep keeping I suggest you start by reading the Sheep Book for Smallholders by Tim Tyne. Sheep breeding generally keeps pace with the seasons: the ewes come into season as the days shorten in Autumn, which means that 21 weeks later their lambs are born to coincide with the fresh grass growth of Spring to support the ewe producing milk for the lambs. A few breeds will breed out of season but the GFD isn't one of them.