In probability theory, the birthday problem asks for the probability that, in a set of n randomly chosen people, at least two will share the same birthday. The birthday paradox is the counterintuitive fact that only 23 people are needed for that probability to exceed 50%.
My dad and his two siblings had the same birthday. It is a weird bit of trivia and I never got to ask my grandparents what the heck was going on with that!
Clearly they had one day a year scheduled for sex