Inspired by simonbengtsson/eventcal.
Copy .env.example to .env.
Go to Facebook → Events → See All, right-click the "Add to Calendar" button and copy the target (e.g. https://www.facebook.com/events/ical/upcoming/?uid=123&key=abc).
Put this link and a random password in the .env.
Test locally with Vercel CLI by running vercel dev and adding the password to the querystring:
http://localhost:3000/all?password=<PASSWORD>
http://localhost:3000/going?password=<PASSWORD>
http://localhost:3000/maybe?password=<PASSWORD>
Fill in your Vercel project's environment variables with the entries from .env.
Add your Vercel project's URL to your calendar application with the password and the optional status parameter: