Some people like to breed for a fall colt. There's this mare that I've been watching on my way to and from work that I've been expecting to drop a foal every day for 2 months. She is just HUGE. A friend of mine was planning on breeding her mare in October one year. I know that most mares stop cycling along about then unless they're kept under artificial light so that might be why you don't see it that often.
Well, I think the main reason, besides the accepted New Year's birthday, is that most don't want a young foal to have to endure winter...
But folk in the South barely know what winter is eh? LOL
Your winter might be a milder time for a young tyke than summer :~D