Birthdays tend to be a special day for every individual, but every so often one encounters a person with the same birthday as them. This might seem relatively unlikely but, for some birthdays more than others, it's quite the opposite. If you've ever wondered how many people share your birthday, look no further.

What Are the Odds?

When it comes down to it, if your birthday falls on any day other than February 29, the odds of sharing your birthday with anyone you meet should be approximately 1/365 in any population (0.274%). Since the world population is estimated at over seven and a half billion,﻿﻿ you should, in theory, share your birthday with over 20 million people (~20,438,356).

However, if you were born on the leap day February 29, you should share your birthday with just 1/1461 of the population as 366 + 365 + 365 + 365 is equal to 1461. Because this day only comes around once every four years, a mere 0.068% of people worldwide claim it as their birthday—that's only 5,072,800 people!

Why Some Days Are More Popular Than Others

Even though logically the odds of being born on any given date seem like they should be about one in 365.25, birth rates don't follow an even distribution—a lot of things affect when babies are born. In the American tradition, for example, a high percentage of marriages take place in June and this leads to many babies born between February and March.

It also seems likely that people conceive children when they're rested and relaxed and/or when options for leisure are most limited. Random natural and unnatural events such as blackouts, snowstorms, and flooding tend to keep people inside and, therefore, increase rates of conception. Holidays known for inspiring warm feelings, like Valentine's Day and Thanksgiving, are also known for skyrocketing pregnancies. In addition, a mother's health greatly affects her fertility, so it makes sense that environmental stresses make conception less likely.

Since the 1990s, several scientific studies have shown that there are seasonal fluctuations in conception rates.﻿﻿ Birth rates in the northern hemisphere, for example, typically peak between March and May and are at their lowest between October and December. Those numbers, of course, vary widely according to the age, education, socioeconomic status, and marital status of parents.

Crunching the Numbers

In 2006, The New York Times published a data table titled "How Common is Your Birthday?"﻿﻿ This table, compiled by Amitabh Chandra of Harvard University, provided data on how often babies are born in the United States on each day from January 1 to December 31. According to this piece, children are far more likely to be born in the summer than any other season, followed by fall, spring, and winter respectively. Early to mid-September features the most common birthdays, though the top most popular day moves slightly from year to year. Right now, this day is September 9.

Unsurprisingly, February 29th is—and probably always will be—the least common or one of the least common birthdays. Outside of that rare day, the 10 most unpopular days reported in this study were holidays: the 4th of July, late November (days near and including Thanksgiving), over Christmas (December 24–26), and New Year's (December 29 and January 1–3), in particular.

Some might suggest that these low popularity birthdays mean that mothers have some say in when their baby is born and prefer not to deliver on holidays. Since this study, more recent data has emerged to confirm that holidays maintain the lowest birth rates and the first ten days in September the highest.﻿﻿