No. In U.S. birth records from 1994 through 2014, 73 of the 365 dates that occur every year had at least 5% more or fewer births than an even daily spread would give them. Using each year's weekday averages as the reference leaves 72 dates outside that range. Five percent is a convenient cutoff for describing the data, not a test of statistical significance.
We gave every day in a year an equal share of that year's recorded births, then added up the years. That works out to about 234,676 births for any date other than February 29.
How far do individual dates differ?
Here are four dates against that even-spread reference. A positive difference means the records have more births than the reference; a negative difference means fewer.
| Date | Recorded births | Equal-day benchmark | Difference |
|---|---|---|---|
| September 9 | 259,228 | 234,676 | +10.5% |
| July 4 | 185,315 | 234,676 | -21.0% |
| December 25 | 138,628 | 234,676 | -40.9% |
| February 29 | 52,561 | 56,119 | -6.3% |
September 9 had 259,228 recorded births, 10.5% above the even-spread count. December 25 had 138,628, 40.9% below it. July 4 is low too. The 73-date tally includes ordinary dates as well as familiar holidays and popular birthdays.
You can check your own birthday to see its historical share and rank. The birth-month comparison looks at whole months after allowing for their different lengths.
What about February 29?
February 29 occurred only 5 times in these 21 years. Across those five days, an even daily spread would give it about 56,119 births. The records show 52,561, or 6.3% fewer. Comparing that raw count with a date that occurred 21 times would exaggerate the difference. We leave leap day out of the 365-date tally for the same reason.
Does the weekday pattern change the answer?
Births also vary by weekday. To check how much the calendar's weekday mix matters, we compared each occurrence of a date with that year's average for its weekday. When a date fell on a Tuesday, we used the average Tuesday in that same year. We then added those yearly references for each birthday date.
With that weekday reference, 72 recurring dates are at least 5% above or below the reference. The even-daily-births reference gave 73. September 9 is still 10.2% above its weekday reference; December 25 is 40.4% below.
This comparison accounts for the overall weekday pattern in each year. It cannot tell us why any date differs, and the change from 73 to 72 does not measure the full role of weekdays. Our weekday study gives the daily averages directly.
Data and calculations
We counted 85,712,738 births on 7,670 calendar days in FiveThirtyEight's pinned U.S. daily files. We used CDC/NCHS records for 1994 through 1999 and SSA records for 2000 through 2014. The overlapping CDC years are excluded. These two sources have different administrative coverage.
For the even-spread reference, we divided each year's births by its 365 or 366 days, then added the resulting daily average for every year a month/day existed. For the weekday version, we calculated each year's average for each weekday and used the matching average for every date. Each percentage difference is (recorded births ÷ reference births − 1) × 100.
The calculation data list all 366 dates and both references. These historical U.S. records cannot show today's or the world's birthday distribution, or why a particular birth happened when it did. See our methodology for source limits. If your question is about the chance that two people share a birthday, read the birthday paradox.