HGG Community Forums

I might be corrected by someone else, but it is every odd month (ie, January, March, May, July, September, and November) and occurs between the days of the 2nd - if you have a game upgrade - and the 3rd - for players who dont have an upgrade - through the 26th I believe.
When you first log into the game, during the first few days before breeding season you will see a large message header that will tell you, essentially: You will be able to breed on the (second or third) day of this breeding season at (a time is designated for when you can begin your breeding). Or pay 50,000hbs to advance your time slot by 1hr.

I hope I got that right and that it helps! :)