What is the most common birthday in the United States?
The single most common U.S. birthday is September 9, with an average of 12,301 births on that date each year across the 1994–2014 series. Eight of the top ten dates fall in September; the only non-September entries in the top ten are July 7 and a small cluster of late-September dates close behind. Mid-September is unambiguously the peak.
The pattern is even clearer once you widen the lens to the top 30: 17 are in September, 5 in December (concentrated around December 18–22), and 4 in July. No other month appears more than twice. Birthdays cluster powerfully in late summer.
| Rank | Date | Avg births / yr |
|---|---|---|
| #1 | September 9 | 12,301 |
| #2 | September 19 | 12,229 |
| #3 | September 12 | 12,224 |
| #4 | September 17 | 12,148 |
| #5 | September 10 | 12,143 |
| #6 | July 7 | 12,108 |
| #7 | September 20 | 12,107 |
The most widely cited explanation for the late-summer peak is conception around the winter-holiday period roughly nine months earlier — a tidy story that lines up with the calendar, but one that's an explanation rather than a proven cause. Whatever the mechanism, the seasonal pattern is real, it's large, and it persists across the entire 21-year window.
The full-year birthday distribution
The chart below plots the average number of U.S. births on every date that occurs every year. Two features dominate: the broad late-summer ridge centered on mid-September, and the sharp, narrow dips on Christmas, New Year's Day and July 4. Everything else is variation around an average of ~11,132 births per day.
The labelled points are the dossier-verified anchors used to validate the build: September 9 (~12,301), December 25 (~6,574), January 1 (~7,792) and July 4 (~8,796). Christmas alone sits about 47% below September 9 — one of the starkest patterns in U.S. natality data.
What is the rarest birthday?
A note on the numbering before we go further. There are 366 possible calendar dates a person can be born on, but only 365 of them occur every year. The tool ranks those 365 from rank 1 (the most common) to rank 365 (the least common day to be born on, which is December 25). February 29 sits apart as the 366th — it isn't ranked by daily average because it only arrives once every four years, and that structural rarity is its own kind of rare.
"Rarest birthday" actually means two different things, and they have two different answers. It's worth being precise because the two figures point at very different mechanisms.
The rarest birthday to have: February 29
February 29 is roughly a quarter as common as any other birthday, because it lands only once every four years (the leap-year rule). That's the answer almost everyone means when they search for "rarest birthday," and it's the right one — the rarity here is structural, not behavioural.
The precise figures: across our 21-year window Feb 29 appeared 5 times and accumulated 52,311 births — a 0.0613% share of all U.S. birthdays in the period, or about 22% of a typical date's share.
We don't give Feb 29 a numeric rank on the chart because its per-occurrence average isn't directly comparable to a normal date's. On the leap years it appears, around 10,462 babies are born on it — a fairly ordinary number — but the share of people who carry that birthday is structurally tiny because the date itself is structurally rare.
The least common day to be born on: December 25
If you exclude leap day and ask which calendar date sees the fewest births when it occurs, the answer is December 25, with an average of just 6,574 U.S. births per year. New Year's Day is next, then Christmas Eve and Independence Day — every one of the bottom four is a major holiday. This isn't a calendar artifact; it's a behavioral one.
| Date | Avg births / yr | Why |
|---|---|---|
| December 25 — Christmas | 6,574 | Holiday — almost no scheduled deliveries. |
| January 1 — New Year's Day | 7,792 | Federal holiday. |
| December 24 — Christmas Eve | 8,069 | Holiday eve — many practices closed. |
| July 4 — Independence Day | 8,796 | Federal holiday. |
| January 2 — holiday tail | 9,307 | First weekday after New Year's. |
| December 26 — holiday tail | 9,543 | Day after Christmas. |
| Nov 27 — Thanksgiving week | 9,718 | The Thanksgiving week (Nov 23–28) averages roughly 9,700–10,100 across its dates. |
| October 31 — Halloween | 9,978 | Small scheduling avoidance. |
| April 1 — April Fools' | 10,300 | Mild but consistent dip. |
You'll see the same shape on every chart for the rest of this page: a holiday is a dip, and a weekday is a peak. The mechanism that produces both is the same one.
Why is Christmas the least common birthday?
In modern U.S. obstetrics, a large fraction of births are scheduled — either induced or delivered by planned cesarean. Those bookings happen during normal practice hours, which means almost none of them land on a federal holiday. The births that do happen on December 25 are the ones that arrive on their own; the holiday effectively strips out everything else.
The same logic explains why every other federal holiday shows up in the bottom of the rankings — New Year's Day, July 4, Thanksgiving — and why the dips have visible "tails" on the days bracketing them (January 2, December 26). Minor cultural dates like Halloween and April Fools' show smaller but consistent dips of a few percent. The established demographic-research framing for this pattern comes from NCHS Data Brief No. 200, May 2015, which documents how cesarean and induction scheduling drive the weekday and holiday patterns in U.S. natality data.
Which day of the week has the most U.S. births?
Tuesday, by a clear margin: about 12,788 babies are born on an average Tuesday in the U.S. Sunday is the trough at 7,610 — about 40% below the Tuesday peak. Weekends average roughly 34% fewer births than weekdays.
The shape is striking: Tuesday through Friday cluster between ~12,304 and ~12,788, Monday trails slightly at ~11,563, and then the weekend collapses to ~8,593 on Saturday and ~7,610 on Sunday. That cliff is the same scheduling effect at work — the births that can be scheduled get scheduled into the business week, and the births that can't follow a much flatter distribution underneath. Weekend dates aren't actually rarer in any meaningful biological sense; they're just less convenient.
Which month has the most U.S. births?
On a per-day basis, September leads, with about 11,764 births per day on average — well ahead of every other month. By total share of births, August (8.90%) and July (8.78%) lead because each contains 31 days; September is third by total share even though it dominates per-day.
The seasonal low is January, with ~10,678 births per day on average — about 9% fewer than September. The full month-by-month picture mirrors the calendar curve: a broad late-summer hump, a long late-autumn decline, and a winter floor. The 12-month ranking and the per-day-vs-total split (September leads per day; August by total because it has 31 days) lives on the Birth Month page.
How many people share my birthday?
For a typical date, about 11100 Americans are born each year — that's the headline answer the tool gives most users. On the rarest non-leap dates (Christmas, New Year's, Christmas Eve, July 4) the figure is between 6,574 and 8,796; in mid-September it's over 12,000.
Multiply any of those by the population alive at any given time and you get a sense of how many same-birthday people are walking around the country today. Birthday Twins estimates that count for any date (about 22 million worldwide, ~930k in the U.S. on a typical date), and the Birthday Paradox page covers the odds of meeting one.