Who shares my birthday?

Someone born on the same day and month as you, in any year. Adam Sandler and Michelle Williams are birthday twins: both were born on September 9, in 1966 and 1980.

In a group of 23 it is 50.8%, counted on births in the United States, a shade above the 50.7% that 365 equal days would give. It passes 99% with 57 people. This is the birthday paradox.

September 9: about 1,020,000 people alive in the United States were born on it, by our estimate. December 25 has the fewest of any date that comes every year, about 608,000, and February 29, which comes once every four years, about 219,000.