Breeding Season
When breeding season starts and ends. What happens when it ends? Foals need to be removed from pasture. Horses age up at the end of the year. What you can still do outside of breeding season. When breeding will open for you, depending on upgrade/purchases. When pastures will open for you.
(Parts of this can be reused in the Game Time vs Real Time subsection in Learning Your Way, or link to here from there.)