I think making it depend to the birthday is also rational. Otherwise there is more possibility of some sort of resource congestion if horoscope compels every user (rather than 1/12th of the users) to do same thing. This is not a big concern in the Internet with few thousands users spread all over the world - but imagine a small village, where there's just 1 horoscope, and it tells everyone to think about failure modes of their plans - they are all farmers, they all think what they're gonna do if crops fail, they all try to buy same equipment at same day.