It would be interesting to have some data like that per country. For example in Japan, I would suspect that the first places are in April and May. The fiscal year beginning on the 2nd of April, couples prefer to be able to put a kid to the daycare in its first year in the 0-year-old class (0歳児) with a kid the maximum older. So if a kid is born on the 2nd of April, he will be able to go to the daycare of 0-year class when he is 11 months and 30 days, while if he is born in March, the kid could just enter the daycare in the 1-year class (1歳児), which is way harder to join because there are less new places in 1-year classes than 0-year classes. Note that the kid can just go to the daycare if older than 6 weeks (I may not recall details correctly here), so it is possible for kids born in February to go to the daycare of 0-year class, but most of his/her classmates would likely be closer to turn 1 once they all join the daycare, which makes for a huge difference at this age.