Is August 31 a rare birthday?

August 31 is a relatively common birthday across the year in these historical U.S. records. It ranks #85 out of 366 dates, where #1 means most common. Within August it ranks #26 of 31. A date near the less common end of a busy month can still fall within the year's top 100.

August 31 had 243,440 recorded births from 1994 through 2014: 0.284% of the sample, or about 1 in 352 recorded births. These figures describe historical records, not the living population or births this year.

How does August 31 compare with nearby dates?

August 30 has more recorded births than August 31, while September 1 has fewer. Later September dates, including September 9, are among the year's most common birthdays. The table gives the counts behind those rankings.

August 31 occurred 21 times and averaged 11,592.4 births per appearance in the baseline. The August guide keeps the full within-month ranking; the September guide explains the next month's pattern. The most-common-birthday guide compares the all-year leaders.

Adjacent calendar dates fall on different weekdays. Since weekday and weekend birth counts differ, the neighboring totals alone do not tell us whether August 31 differs from other dates sharing its weekday.

What changes when we match weekdays?

For annual comparisons, we use SSA U.S. records from 2000 through 2014. Each August 31 is compared with eligible dates one and two weeks before and after it in the same year. Actual and standard observed federal holidays, Christmas Eve, December 26 and the Friday after Thanksgiving are excluded as controls.

Across these 15 years, August 31's births are 0.9% below the comparison average. It is lower in 11 years and higher in 4. The overall gap is small, and individual years differ.

We average each year's eligible controls first, then divide the sum of August 31 births by the sum of those annual means. Matching weekdays leaves other seasonal differences. This comparison cannot establish an end-of-month effect.

Why exclude a September comparison date?

The candidate controls are August 17, August 24, September 7 and September 14. In 2009, September 7 was Labor Day, so that date is excluded and the other three supply the year's comparison mean. OPM's federal holiday-designation fact sheet places Labor Day on the first Monday in September. The exclusion follows a calendar rule; it does not assume a particular hospital schedule.

Using only seven-day controls produces -0.4%; using only fourteen-day controls gives -1.3%. Both checks retain 15 years. The precise gap depends on the comparison window, but neither gives August 31 a large difference from those nearby matched weekdays.

Sources and limits

The baseline uses daily U.S. birth records compiled by FiveThirtyEight: CDC/NCHS for 1994 through 1999 and SSA for 2000 through 2014. We exclude overlapping CDC years; the sources have different administrative coverage. Annual weekday comparisons use SSA alone.

Overall rank compares total births across 366 dates. Within-month rank compares August's 31 dates. Birth share divides this date's count by all recorded births in the baseline. None is a worldwide estimate or a count of everyone alive with an August 31 birthday.

The methodology explains those measures and source boundaries. Use the birthday checker to compare August 31 with another date.