Ever walked into a room and realized three different people are all celebrating their big day on the same afternoon? It feels like a weird glitch in the matrix. Honestly, if you're hanging out with a group of friends in mid-September, the "Birthday Paradox" starts to feel less like a math theory and more like an expensive week of buying drinks.
So, what is the most common birthday in the United States?
September 9. That’s the one. If you were born on this day, you share a cake-cutting schedule with about 12,301 other Americans born every year, at least according to data crunching from the National Center for Health Statistics. It’s a crowded club. But why? Is there something in the water in December, or is it just a massive coincidence?
The September Spike is No Accident
If you look at the top ten most common birthdays in the U.S., September basically runs the table. Aside from a random appearance by July 7 (likely due to parents wanting to avoid a July 4 holiday birth), the list is a wall of September dates.
- September 9
- September 19
- September 12
- September 17
- September 10
It's a bit ridiculous.
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When you do the "backwards math"—counting nine months back from September—you land right in the middle of the holiday season. Think Christmas and New Year’s Eve. It’s cold outside. People are off work. There’s a lot of "holiday cheer" (and maybe a bit too much eggnog) going around.
Pediatricians like Dr. Whitney Casares have pointed out that during these festive stretches, people are simply more relaxed and spending more time together indoors. Biology also plays a part. Some studies suggest that sperm quality actually improves in cooler temperatures, making conception a bit more likely during the winter months compared to the sweltering heat of August.
Why Holidays Are Birthday Deserts
On the flip side, the rarest birthdays are almost always holidays.
December 25 (Christmas Day) is the least common birthday in the country, followed closely by January 1 (New Year’s Day).
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This isn't just because babies choose to avoid the "one big gift for both" struggle. It’s because of how much we control the timing of birth now. With C-sections and medical inductions making up a massive chunk of U.S. deliveries, doctors and parents generally don't schedule "elective" procedures on major holidays. Nobody wants to spend their Christmas morning in an operating room if they can help it.
Even February 29—Leap Day—is technically more common than Christmas. That’s wild. Since Leap Day only happens every four years, you’d expect it to be the absolute rarest, but because it’s a "normal" workday for hospitals, more babies are born on that one day than on a typical Christmas Day.
The School Year Strategy
There’s another layer to the September 9 dominance: the school calendar.
Many parents are aware of age-cutoff dates for kindergarten. Being the oldest in the class is often seen as a competitive advantage in both academics and sports—a concept popularized by Malcolm Gladwell in Outliers. While not everyone is "planning" their child's birth to the exact day, there is a subtle pull toward having a late-summer or early-fall baby so they aren't the tiny kid struggling to keep up with classmates who are nearly a year older.
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Fun Fact: The 9/9 Celebrity Club
If you’re a September 9 baby, you’re in pretty famous company. You share a birthday with:
- Adam Sandler
- Hugh Grant
- Michelle Williams
- Michael Bublé
- Colonel Sanders (Yes, the KFC guy)
Does It Actually Matter?
Kinda, yeah.
For hospitals, knowing these trends helps with staffing. Labor and delivery wards are notoriously slammed in September. For you? It mostly just explains why your social media feed is a non-stop stream of "Happy Birthday!" posts the second the Labor Day decorations come down.
If you’re looking to plan a party and your birthday falls on September 9, book your venue early. You’re competing with a lot of people.
Next Steps for You:
Check your own birth date against the national rankings to see where you land. If you’re a "Holiday Baby," take comfort in knowing your birthday is statistically unique. If you’re a "September 9-er," maybe it’s time to start a club—you’ve definitely got the numbers for it.