Um im not sure I understand your question but the sheds are open from February to July. The universal "birthday" for all thoroughbreds is January 1st of their foaling year, so the concept is to have the mare foal as close to the January 1st date without giving birth before that. For example, if a mare foals on Dec. 31 st, then the next day, Jan 1st, that foal would be considered one year old.

And the September part of your question is throwing me. The only thoroughbreds that will foal any where near September are those who are breeding for sport horses... not race horses.