I posted this question earlier this year. Link If an app on your Mac stops responding, closes unexpectedly, or won’t open was not helpful as this is the calendar app that comes with Mac OS. I am running Big Sur 11.5, iMac (Retina 5K, 27-inch, Late 2015).
See the article Repair a storage device in Disk Utility on Mac - Apple Support
Create a new admin account in users and groups Set up users, guests and groups on Mac - Apple Support
Uninstall vpn services , third party apps / cleaners / softwares / anti virus from Mac https://support.apple.com/en-in/guide/mac-help/mh35835/mac
See the article Repair a storage device in Disk Utility on Mac - Apple Support
Create a new admin account in users and groups Set up users, guests and groups on Mac - Apple Support
Uninstall vpn services , third party apps / cleaners / softwares / anti virus from Mac https://support.apple.com/en-in/guide/mac-help/mh35835/mac
Mac OS 'calendar quit unexpectedly'
