Overtaking a city?

In attempting to overtake a city I found out you can only attack somebody 5 times in one day ? Is it based on 24hours or after midnight can I attack again? Do you have to team up with another person occupy the city?

It is easiest to team up with someone. But it does just reset at 00 GMT. So if you start the attack shortly before, then you should be able to continue the attacks.