tywkiwdbi which in turn had seen it on TheDailyViz. As a sort-of sheep farmer I know that it's effectively impossible to have lambs born before January or after April: the sheep only come into estrous so as to have Spring lamb - for very good evolutionary reasons that will be apparent to anyone who's watched Attenborough on The Box. So it was news to me that there is a seasonal trend in human births with a peak in late summer - 16th September is the commonest birthday.
Minnesotastan who writes tywkiwdbi noted the absence of births around Christmas and the 4th of July. There are plenty of births on those days, of course, but they are disproportionately rare on these holidays (the data are exclusively US). A quite staggering proportion of births in the US are carried out by C-section - much 'safer', much less likely to involve litigation. So the birthday is largely under control of the parents and their Ob&Gyn team and nobody wants to ruin their holidays.
But I was also intrigued to note a blip in the opposite direction on 14th Feb and 17th March. Ahhh, how romantic, to arrange to have your child delivered on Valentine's Day. My Granny was born on 14 Feb 1892 (and lived on into a third century to die in 2001) and her parents called her Lily Valentine in honour of the day. I'm assuming that the up-blip on 17/Mar is due to patriotic Irish Americans wanting to a) name their child Patrick Jr and b) not forget when his birthday is. But I can't find a similar Just-So-Story to account for the excess births on 3rd March - St Cunigunde of Luxemburg's Day? Hinamatsuri ? Alexander Graham Bell's birthday (a LOT of people work for Ma Bell) ?