Amitabh Chandra, a professor of public policy at the Harvard University, earlier published a data table detailing how many babies were born in the United States on each date between 1973 and 1999. September 16th happens to be the most common birthday while December 25th is the least popular birthday
Matt Stiles, data journalist at NPR, converted this NYT table into a static heatmap for easy visualization (the darker the color, the higher the probability of births happening on that date) and Andy Kriebel made an interactive heatmap of the same data using Tableau – this version lets you hover over any cell and you can see the underlying data.
Going back to the data, it turns out that September is the most popular month to have babies and, according to Wikipedia, this is likely due to the fact that the holiday season is nine months before.
Related reading: Do Check Google on your Birthday