How to Find Out About Events in Toronto
BlogTO has an events listings page, and every Wednesday, they post of their picks of best things to do on the weekend.
Now Magazine has an events listing, which they maintain, and which people organizing events can post to.
Facebook, through both the notifications you get of a friend is interested in an event nearby, plus if you're following a page, you will get notified of an event posted there.
The City of Toronto has a Festivals and Events Calendar.
Meetup is a good way to find regularly-occurring events in Toronto. Your mileage may vary for other locales, though.
Toronto Public Library posts event listings:
For bands you know you like, sign up for an account at Songkick or Bandsintown, and follow your favourite musical acts. At Bandsintown, you can ask a band to come to your city, but I don't know how/if that works. I click "Come to Toronto!" for every band I start to like.
There's nothing like Events12 for Toronto. That site, pre-pandemic, would have a rolling year-long calendar with the general schedule for future events. It would give you a chance to pick a good time of year to visit an American city. As of 2022, they are only updating the Seattle page. They should have a page for Toronto, North America's 4th largest (and therefore 4th best) city.