problem with Calendar on Big Sur

Hello, Bruce!

After reviewing your post, it seems like you are running into some issues with your Mac calendar app. We definitely understand that not being able to manage your calendars as expected can be a concern and we'd love to help. Let’s start our investigation by determining exactly when this happens. First, test for this behavior in safe mode: Start up your Mac in safe mode.

Next, see if this behavior persists in a second user account on your Mac. If you don't have one you can use, please follow these steps to make a test account that can be deleted later: Set up users, guests, and groups on Mac.

Please let us know if you have follow-up questions about this issue, or if the issue remains unresolved. Have a great day!

Thanks for getting back with us, Bruce. Just to clarify, the same behavior occurs in both safe mode and on a second user account? We want to make sure of that, as this will take the troubleshooting in a different direction.

Also, is your Mac backed up to Time Machine or another source? When did this issue begin? Do you remember any software or hardware changes that took place right before the issue started? For example: new apps installed or deleted, physical changes to the device, software updates, etc? Are you able to test your Mac on a different network?

Shoot us over a reply when you have a moment, with these answers, and we can continue. Thanks and take care.

One thing you could try is to sign off Apple ID on your Mac, restart it and sign on again. If this fails, turn off Calendar synch on your Mac and turn it back on.