What's the most common birthday?

September wins in most countries

The busiest birthday falls in September in 7 of the 13 countries: September 19 in England and Wales, September 24 in France, September 18 in Hungary, September 19 in Mexico, September 20 in the Netherlands, September 27 in New Zealand and September 9 in the U.S. Count back about 38 weeks from mid-September and you land right on the Christmas and New Year holidays, when people have time off together.

Even where Christmas is in summer

Here's our favorite clue. In New Zealand, Christmas falls in the middle of summer, and yet all 10 of its most common birthdays are in late September or early October too. So the September crowd isn't about cozy winter nights: it's about the holidays.

Summer winners in Belgium, Denmark, Germany and Slovenia

July 23 in Belgium, August 18 in Denmark, July 7 in Germany and August 8 in Slovenia top the list, with July and August dates making up 6 of the top 10 in Belgium, 10 of the top 10 in Denmark, 6 of the top 10 in Germany and 3 of the top 10 in Slovenia. Norway sees the same summer peak today, after decades of spring peaks (Statistics Norway).

Brazil breaks the September rule

Brazil's busiest birthdays come in March, April and May instead: all its top 10 dates, led by March 10. Like New Zealand it's in the southern hemisphere, yet the two countries peak half a year apart, so the weather alone can't explain either pattern.

In Japan, babies wait for the holidays to end

Japan's most common birthday is January 5, and it's no accident. During the New Year holidays, January 1 to 3, births drop 30% to 35% below a typical date, then January 5 jumps to 17% above it. Golden Week does the same in spring: May 3 to 5 are 22% to 26% quieter than usual, while May 1 and 2 are 11% and 14% busier. Planned births are simply booked around the holidays. In fact, all ten of Japan's rarest birthdays fall on the New Year break or on a national holiday, from Labour Thanksgiving Day (November 23) to National Foundation Day (February 11).

Holidays empty the delivery rooms

The quietest birthday is a holiday everywhere: December 25 in Belgium, Brazil, France, Germany, Mexico, New Zealand, Slovenia and the U.S.; December 24 in Denmark and Hungary; December 26 in England and Wales and in the Netherlands; and January 1 in Japan. In Denmark and Hungary it's Christmas Eve, the evening families open their presents.

National days show up too: July 4 is 363rd of 366 in the U.S., Belgium's National Day, July 21, is 360th while July 23 is its busiest birthday, and New Zealand's Waitangi Day and Anzac Day are 360th and 362nd. Mexico's Independence Day, September 16, is 295th even though it falls in the middle of the country's busiest month, with September 17 in 12th place, and Mother's Day, May 10, is 360th. In Brazil, April 20 is the 7th most common birthday, and the next day, Tiradentes Day, is 312th.

Some winners are clear, others are a photo finish

To see how solid each first place is, we ranked the 12 countries with year-by-year records again, leaving out one year at a time. July 23 in Belgium (13 of 15), September 18 in Hungary (13 of 15), January 5 in Japan (13 of 15), September 20 in the Netherlands (13 of 15) and August 8 in Slovenia (14 of 15) win almost every time, and September 24 in France (9 of 15) and July 7 in Germany (4 of 6) win most of the time. But first place is a toss-up in Brazil (March 10 wins 3 of 15 tries), Denmark (August 18, 3 of 15), England and Wales (September 19, 6 of 15), Mexico (September 19, 6 of 15) and the U.S. (September 9, 8 of 21): several dates are so close that the winner depends on the years you count. The quietest date is much steadier: in 10 of the 12 countries it comes last in every single test, while in Hungary December 24 and December 25 take turns and in Slovenia December 25 gives up last place only 3 times, to November 1 and December 26.

That's why the date you'll often read for England and Wales, September 26 (the ranking for 1995 to 2014), and today's September 19 are both fair answers: they belong to the same late-September pack.

Same date, different story

Some birthdays change completely when you cross a border. December 28 is the 28th most common birthday in the U.S. but 362nd in England and Wales: American births pile up in the last days of December, and economists have linked part of that to tax rules that reward a baby born before January 1 (Dickert-Conlin and Chandra, 1999). July 18 is Denmark's 2nd busiest birthday but New Zealand's 356th. And January 5, first in Japan, is 241st in the U.S.

Weekends are quieter everywhere

An average Sunday has fewer births than an average Tuesday in every country with daily data: 43% fewer in Belgium, 41% fewer in the U.S., 39% fewer in Japan, 30% fewer in Brazil, 29% fewer in Hungary, 27% fewer in Slovenia, 26% fewer in Mexico, 24% fewer in the Netherlands, 21% fewer in Germany, 20% fewer in France, 19% fewer in Denmark and 18% fewer in England and Wales. That's why the weekday you were born on matters so much for how crowded your exact birth date was.