I'm not sure if this has been answered before, but I've skimmed through the other pages.
Each household is allowed 10 days vacation. Vacation can be accrued and passed on to the next household. So, if you never take a vacation in your starting house, when you begin playing your 2nd house, you then have 20 days vacation available. For every Sim that goes on vacation that counts against the vacation total—i.e. 3 Sims go on vacation for 3 days = 9 vacation days.
I was wondering if the time off for maternity leave + marriage counted as vacation? It sounds like this only applies to people with the World Adventures expansion pack but I wanted to double check. Now that I"m typing this out, though, it sounds like the vacation thing is totally separate. XD