How do I turn the calendar feature on?

Calendar events are a Premium feature. On the free plan, switching them on opens the subscription page.

1. Open the app, tap the gear icon in the top-left, and tap Data and Notifications in the menu.
2. Scroll to the Calendar card.
3. Tap Allow on the "Calendar permission" line. Your phone will pop up the system prompt. Choose Allow there too.
4. Now switch on the "Calendar events" toggle just above it. That is it.

The next time you open an evening check-in, you will see a page called From your calendar with everything that was auto-saved.

I tapped Don't Allow by mistake

That is fine. Tap Allow on the same row again. If your phone no longer shows the prompt, open your phone's own Settings app, find MigraineMe, and switch the Calendar permission on there. Then come back to the app and the toggle will be ready.

I want to turn it off again

Flip the "Calendar events" switch back off on the same Data screen. New events stop being read. The From your calendar page can still show in the check-in while your phone allows calendar access, but nothing new is added to it. If you want the page gone too, switch the Calendar permission off for MigraineMe in your phone's Settings app. Anything that was already logged stays in your log. It is not deleted just because you turned it off.