Are all horses born on the same day?
All horses born in the same year share their official birthday as the 1st of January. When racing as two-year olds, a horse born in the early months of the year is likely to be stronger and more mature than one born later, despite officially being the same age.
American thoroughbreds all celebrate their 'birthday' as January 1 of the year they were born - so that all 3-year-olds remain 3 year olds the entire calendar year. This isn't true of all breeds nor all countries. Horses are born all year, early spring is most frequent and safest.