Apparently the most common birthday in the U.S. is October 5th. That lines up nicely with New Year’s Eve conceptions (since gestation is actually closer to 40 weeks than to 9 months, if I recall right, and months have different numbers of days anyway.)
The least common birthday is May 22. I guess it’s too hot in August.
These may change in the future, due to modern conveniences like air conditioning and so forth.
Anyway, I did find a list of the most/least common birthday months, but it didn’t adjust for number of days in the month. So it was pretty useless. Of COURSE February is the least common birthday month. *mutter*
But they did show August as #1 and July as #2, which doesn’t sound right to me at all. Anyway, as I said, not terribly useful if you don’t adjust for the length of the month.