If you’ve played The Sims 4 City Living for more than 1 Sim day then your Sim has probably made some acquitances/friends around the town. The more Sims you know the better odds are of getting the following message on your phone:
Your Sims will be able to kill off their free time by going to certain ”rabbit hole” events around the town such as the opera, circus and an underground concert.
If you choose to go to such events, you’ll be asked again around the prompted time you got in the first message asking if you want to go:
Depending on your Sim these events will have a certain impact on their mood: