Avoiding the birth of a child right before winter would be advantageous for its survival… winter being (on average, not all regions/locales obviously) the hardest month to survive evolutionarily speaking this would be a beneficial trait and be likely to stick around. Ideally you want your early hominid babies at the end of winter through spring and summer so at worst they are though the first few fragile months that put the most drain on the mother, before the harshest winter weather sets in.
While not globally consistent due to regional weather differences it makes sense for there to be a broad average seasonal benefit.
Except those “early hominid” lived in Africa, which isn't known to be the place with the harshest winter. And the fact that humans living in temperate climates and humans in tropical areas belong to a single species makes me believe that adaptation to winter wasn't that big of an evolutionary factor.
Now you assume this wouldn't be something that is common for most mammals. In places where there is always sun there is no reason to evolve away people having more sex when there is sunny.
Meanwhile here in northern Europe we get markedly more children during March-April. Which of course could be a sociological thing, or depend on something else, but still.
I believe the phenomenon in Scandinavia that more children are born in spring is partly due to families performing economic optimization wrt. timing of kindergarten and parental leave.
-out of pocket maximum expenses are capped per calendar year. Typically, healthcare expenses for births happen in the latter half than former, and so you can potentially save a few thousand dollars, and even up to $17.4k (legal max). Post birth complications are plenty, and pelvic floor physical therapy does wonders. Not worrying about costs for these is a huge benefit.
-new resident doctors start in June, so you have less risk of a brand new doctor in April/May when they would have had 10 months of experience
-kid would be 6 months of age by winter season, so eligible for flu vaccine, and now Covid vaccine too during their first winter
-lots of schools have hard cutoffs at Aug 31 or around there. Birthdays before then, kid is in school, and after that, kid has to wait another year to start school. This could mean 6+ months of fewer daycare expenses, easily worth $10k
While not globally consistent due to regional weather differences it makes sense for there to be a broad average seasonal benefit.