Why are most kids born during July/August/September ?

[BigSkyBob]

Here is a better theory:

Historically, food was most plentiful starting in the spring, and going into the harvest.  That is, women would be thinnest at the start of spring, and have the greatest fat reserves around harvest time. Even today, fresh fruits and vegetables are more ready and cheaper in that spring-to-fall time frame.

[BigSkyBob]

If it's evolutionary, shouldn't it match to seasonal variations in East Africa?

Northern lattitude populations may have diverged. East Africa, probably, has an optimal time of conception based on factors such as rainfall patterns, animal migration, river levels, etc.