makeworld (offline name Cole) has a similar effort to this. Please take the time to visit his page as well.
General:
BlogTO has an events listings page, and every Wednesday, they post of their picks of best things to do on the weekend.
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.
Now Playing Toronto lists events of all kinds in Canada's largest (and therefore best) city.
Toronto Public Library posts event listings:
Todotoronto (To-Do Toronto)
Toronto Design Directory has an events calendar.
Young W has a Toronto events listings page.
Movies:
TorontoPlex is a city-wide listing of films. Follow them on Twitter and Letterboxd.
Concerts
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.
Historically, Pollstar (their Toronto, ON page) has been good to me. I don't use it much these days.
Other:
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.
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.