Find out who organized a protest that you would have wanted to join, start following them on social media, they will usually announce any upcoming protests there.

You can also try Googling to find organizations in your area that match your ideologies {ideology} + {location}, e.g. “BLM San Francisco” and sign up for newsletters.

In the beginning it will be a lot of active looking but once you start following the right accounts on social media and you’ve signed up for some newsletters/mailinglists, you should be able to passively consume what’s happening next and where.