Many of my friends have birthdays in the next few weeks. This fact prompted a discussion about the uniformity of birthdays. In Outliers, Gladwell makes the case that birthdays of a group of individuals may appear skewed for subtle reasons; however such results shouldn't hold for the populace.
This question is easy to answer with a bit of Googling. A Dartmouth professor has precisely the required data --- though only for a single year.
I used R to make a quick display of the data.
No, that isn't a data problem. There really are two groups of birthdays.
While looking for the overall date of birth data, I discovered another file from the CDC that explains the effect.
The data in that file show many fewer births on weekends compared with weekdays. This effect is precisely what we see in the plot, which R helps us validate.
This analysis was good enough for my own personal edification. There is still a bit of work left to make these claims statistically valid, but that isn't my point here.