@Sturgis,
My intent was to start a discussion. Feel free to pick it apart. I imply no scholarly research. Sorry to say that if you're looking for exacting info, I confess to just hearing this on a news program about 20 minutes ago. I can no more quote a source or verify this than I can recite the entire Gettysburg Address verbatim.
Being that I was born on Sept 20th, I have a natural curiosity about the subject. I've reard the Sept 15-20 timeframe quoted a lot as being the most popular dates. I recall in my grammar school calsses quite a few people born on or near my birthdate. I had one girlfriend born on the exact same day/year and a sister-in-law orn the day after (diff year). Also I've a close friend born on October 2nd.
However, these anecdotal occurences neither confirm nor deny October 5th date as being the most popular day for a birthday.
It would be interesting to see if someone can come up with a more definitive
answer. Of course the climate at this time of the year probably would influence the result too. So let's limit the geo region to North American continent.
{Edit: Here's one website that claims that day is today from www.americanbaby.com:}
http://answers.yahoo.com/question/index?qid=20061005083222AAvOF2a