During the equine breeding season in the bluegrass state, pastures and fields are filled with adorable new foals alongside their mothers. The first foal of the season is always a much-anticipated event in the bluegrass state. For thoroughbreds, each horse has the same birthday, January first. There are two main reasons for this. First, a mare’s gestation period is about 11 months. Secondly, the horse racing industry organizes races by age group. Therefore, an "older horse" has more of an advantage over its competitors.
Thoroughbred horses race by age. Two-year-old colts and fillies race against other two-year-olds, three-year-olds race against other three-year-olds, and so on.
Obviously, a foal born in December would be at a great disadvantage at the races since it would be slightly more than a year old when it turns two. The breeding season runs from February to July 4, roughly, to accommodate the gestation period of the mares.