One stallion I had could be in the arena 5 acres away from us in the house and still scream loud enough to make you grind your teeth. My husband HATED that horse just because of all his bellowing. Good thing he was only a boarder, when he left I decided if I ever had a stallion I'd teach him not to scream. It's much more peaceful and like one poster said, they seem to always cut loose with a big one, right in your ear. It can be downright painful.
We have things at shows called Futurities, competitions for young horses. So, since they start as weanlings, being born as close to Jan 1 as possible is the goal. All horses here age 1 year on Jan 1, so the horse born on Dec. 17, turns 1 year old on the first of Jan, even though he's only a couple weeks old he's considered a yearling. The horse born on Jan 2 who turns 1 on the following Jan 1, is now actually 1 year old. So, my first futurity for my weanlings is in September and my horses born in late Jan or early Feb, will have size and maturity over horses that were born in say, May or June. By the 2nd year, they've all pretty much caught up to each other so it no longer is an advantage but that first couple of years it can make quite a difference.
If I weren't breeding for futurities, I'd breed in April, May, June to have the babies born during the nicest part of the year. Having them born in Jan/Feb means having to heat the barn most years (might not this year, it's going to be 65 F today, not cold at all) and blanketing the foals a lot.