Launching the Notes app subsequently launches the Calendar App as well Whenever I launch the Notes app, the Calendar app launches as well. I use the Notes app a lot and this can be very frustrating. I updated to High Sierra 10.13.5 to see if it would solve the problem, to no success. When calendar opens it asks something about subscribing to a calendar. The message is shown I am on an iMac.

How can I solve this problem?
You should check to see if there is a webcal URL in a note somewhere in Notes.app.

If there is, Notes will open Calendar.app and ask you to subscribe. If you don't actually want to subscribe to this calendar, just delete the webcal URL in the note (or delete the note entirely) and Calendar should not open again.