Your racehorse's first birthday is one month to the day that it was born. For clarity, if your racehorse was born November 6th, then its first birthday would be December 6th. However, there are exceptions to racehorses born on the 31st. For example, if your racehorse was born on January 31st and the following month finishes on February 28th, then its first birthday will be the 28th because that is the last day of the month.