Across 85,712,738 historical U.S. births from 1994 through 2014, December has the widest gap between its highest and lowest annual birthday dates: 82.8%. March has the narrowest, at 5.0%. The table lists both endpoint dates for all 12 months. February 29 has February's lowest total, but it occurred only five times.
Rarest and most common dates in each month
Each row compares dates within one month by total births. The spread shows how much higher the largest count is than the smallest. February has no spread in the table because its lowest total is February 29, which did not occur every year.
| Month | Most common | Births | Fewest births | Births | High-to-low spread |
|---|---|---|---|---|---|
| January | January 24 | 233,021 | January 1 | 164,362 | 41.8% |
| February | February 14 | 245,412 | February 29 | 52,561 | See leap-day note |
| March | March 21 | 235,883 | March 13 | 224,642 | 5.0% |
| April | April 4 | 236,536 | April 1 | 217,042 | 9.0% |
| May | May 23 | 243,051 | May 26 | 218,924 | 11.0% |
| June | June 27 | 244,291 | June 8 | 232,223 | 5.2% |
| July | July 7 | 255,094 | July 4 | 185,315 | 37.7% |
| August | August 8 | 252,122 | August 3 | 238,714 | 5.6% |
| September | September 9 | 259,228 | September 1 | 230,216 | 12.6% |
| October | October 1 | 247,196 | October 31 | 210,629 | 17.4% |
| November | November 21 | 244,132 | November 27 | 205,164 | 19.0% |
| December | December 20 | 253,354 | December 25 | 138,628 | 82.8% |
This table compares birthdays, not whole months. See the birth-month comparison for each month's total and average births per actual day.
How should February 29 be compared?
February 29 has the fewest total February births: 52,561. It occurred 5 times in these 21 years; every other February date occurred 21 times. A year without February 29 contributes no observation for that date. The U.S. Naval Observatory explains the leap-year rule.
Among the dates present every year, February 13 has the fewest February births, at 223,590. February 14 leads. Its count is 9.8% higher than February 13's. The February guide ranks all 29 dates. The rarest-birthday guide compares leap day with the rest of the year.
What does the spread measure?
The percentage is the high date's count divided by the low date's count, minus one. Every date used for these percentages occurred 21 times. For December, the table compares December 20 with December 25. For March, it compares March 21 with March 13.
The gap describes only the two endpoints. It does not measure how evenly births are spread across all dates in a month. These records also cannot explain why one particular date has more births than another.
Source and limits
We combined CDC/NCHS 1994 through 1999 and SSA 2000 through 2014 daily U.S. birth files compiled by FiveThirtyEight. We left out overlapping CDC years. The sources have different administrative coverage, and their counts do not estimate worldwide births or the birthdays of people alive today. Read the methodology for how we calculate ranks.
Use the birthday checker for your own date. The December and March guides show every date between their endpoints; the guide hub has the other months.