Every Birthday on New Year's

Thoroughbreds (TB) always have their birthday on New Year's. How so? Well, in racing, horses race according to age. For instance: the Kentucky Derby is a race for 3 year old TB's. So if a TB is born on January 5th in 2013, he's raced against all other horses born in 2013, and his birthday is said to be January 1st.
TB mares are usually bred earlier in the year, some time soon after January 1st. They typically foal one year later - the goal is to have a TB born as early in the year as possible, so that he has a better advantage.
For example: say a colt is born on December 31st, 2013. He would be racing against all the other TB's born in 2013, which could be up to practically a year older than him.
But if a colt is born on January 1st (the goal for breeders), 2013, he would be the oldest age possible for his age group; therefor, a great advantage.
The older a horse is, the more training he's had.

Here's Mine That Bird racing in the 2009 Kentucky Derby. The gelding won the race, despite the loss predictions. His story is coming out in the movie 50 to 1 (his odds for the race) on March 21st, 2014.