Hi, Stoorsooker.
I'm glad you decided to reach out on the Apple Support Communities forums today. I understand that you’re not able to get either of your Macs to retain the setting when you select “Show Birthdays calendar” as outlined in this guide:
Show or hide the Birthdays calendar on Mac
Have you tried any basic troubleshooting to see if you can get the calendar to stay selected between sessions? If not, I would try each of these and test between each step to see if it stays selected after closing the Calendar app.
How to turn your Mac on or of
Use safe mode to isolate issues with your Mac
Set up users, guests, and groups on Mac
How to update the software on your Mac
How to upgrade to macOS Catalina
If you’re not able to upgrade to macOS Catalina, reply here with the type of Mac you have. It will help me (or another Apple Support Communities user) guide you to the latest compatible version of macOS for your device(s).
Find the model and serial number of your Mac
Please only post the model here. Your serial number should never be given in a public forum.Â
Best Regards.