Birthday guides
and research
Explore the calendar behind your birthday. Each guide explains what the birth records show, how we compare dates, and where the numbers come from.
Birthday guides
Look up dates and compare months using the historical U.S. baseline.
The most common birthdays
Explore the full birthday ranking, September 9’s lead, and the cluster of September dates near the top.
The rarest birthdays, including leap day
See the least common dates and what changes when you count births per actual February 29.
Birth months and month length
Compare monthly totals with daily averages to see how an extra calendar day affects the ranking.
Original research
Original comparisons with their source periods, calculations, and limitations.
Birthday paradox: what are the odds of a shared birthday?
Compare shared-birthday odds for any group size with a birthday paradox calculator. See how the classic model compares with historical U.S. birth records.
Which holidays have the fewest births?
Christmas has the fewest births among four selected fixed-date U.S. holidays in SSA records from 2000 through 2014. See the counts and nearby-date comparisons.
Are more babies born on weekdays or weekends?
Weekdays averaged more births per day than weekends in U.S. SSA records from 2000 through 2014. Tuesday had the highest average and Sunday the lowest.
For source files and definitions, read the methodology .