Sharing your birthday with someone else you know can be fun — or stressful. You might get double the party, but chances are you’ll have to share the spotlight.

This weekend, millions of Americans may have that feeling when the country’s most popular birth date comes around. Birth data shows that Sept. 9 is the most common birthday in the U.S., and September is the busiest month for births overall.

There have been several reported data sets that offer a picture of which days and months are the most common for births. Researchers at Harvard University examined births between 1973 and 1999, and found that the most common birth date for those years was Sept. 16, the New York Times reported in 2006.

But more recently, Matt Stiles at The Daily Viz created a visualization based on data FiveThirtyEight compiled on births from 1994 to 2014, and that’s where the updated date of Sept. 9 comes from. The data is from the National Center for Health Statistics and the Social Security Administration.

No matter which data set you look at, September is a popular time to give birth. The newer data shows that 9 of the top 10 days to give birth fall between Sept. 9 and Sept. 20. Given that pregnancies last an average of about 38 weeks, this means many people are conceiving in December, around the holidays.

On the other hand, winter months like December, January and February are among the least common times for babies to be born. In fact, Dec. 25 and Jan. 1 are the least popular dates in both data sets. July 4 is also a very uncommon birthday. This likely suggests parents are planning their children’s births, Stiles notes on his site, or that doctors take vacations and try to schedule births on non-holidays.

Check out the full ranking of birth dates and a data visualization of their popularity on The Daily Viz.

Write to Abigail Abrams at abigail.abrams@time.com.