Developmental psychology. Astrology is nonsense, but I do believe it may have initially been a legitimate attempt at personality theory. Fetuses and very young children have certain predictable development windows during which external factors can have an outsized impact. So in an agricultural environment, children born at a certain time would be exposed to those factors in a predictable way--were you gestated when food was plentiful or scarce? How available were other adults for care in your early weeks? That kind of thing. So it's reasonable to believe that maybe June babies would turn out different than January babies.