Well along with any star-sign related things it is completely absurd. With 6 billion people in the planet it's quite improbable that they would all have the same day and act the same just from a dozen different types, based solely on when they where born.

There are two major defining factors which contribute to how someones personality can develop, and astrology is not one of them. You are born with possible traits inhereted from your family and develop based on how you are raised, you don't somehow magically gain your personality based on when you where born.