Many horse breeders breed in late winter or early spring, either because of tradition or registry rules regarding horses ages. In reality, there are many advantages to breeding horses to be due in mid to late summer, particularly in regions with mild winters:
Many registries consider ALL horse's birthday's to be Jan 1. So a horse turns one year old whether they were born the previous november, or the previous March. For show purposes, breeding for an early due date allows your foal to be larger and more mature for halter showing, and/or be more mature in time to be trained for two-year-old futurities. However this is not the rule for many breeds, and for horses who may not be broke to ride until 3-5, or who may never be shown, this reasoning does not apply.
When age isn't a factor, summer is the optimal time for foaling horses. Mares foaling on warm summer nights do not need to be put in stalls in the evening. Foals born outdoors take their first breaths of fresh air instead of straw dust. And foaling outside eliminates the risk of the mare foaling against a stall wall and injuring or killing the foal. (Note that there is an elevated risk of predators attacking a foal when born outdoors, but nature intended foals to be born outdoors and most horse-properties aren't overrun with natural predators)
late summer grass has more good nutrients and less sugar than early spring grasses. Summertime grass better nourishes a lactating mare and foal taking their first bites of grass.
There are lots of good reasons to breed in the mid to southern states later if (and it's a big if) you have the facilities to take care of your horses in the event of an early or particularly harsh winter.