I usually wait, until a friend texts my Sim & invites him specifically to the festival. That seems to make it more likely, that it will be set up to the festival. The festival seems to set itself up a bit later in the evening, so if you go, as soon as you get the notification, it might not trigger until later. Sometimes, when you're on the lot, a Sim will text you. You won't travel, but the Sim will arrive & trigger the festival upon their arrival.