How do you go the next day on animal crossing new leaf?

You have to wait a real day. The game works on real time and has created several consequences to "time travelling" (continuously changing the time/date in the settings). If you really want to "time travel" to the next day, you have to select "before I go...","I need help first", ect. while talking to Isabelle in the pre-game screen after selecting your player. You then select "Set the time" and change it to the next day.