Calendar quits immediately after it starts

Hello there,

Thanks for connecting with Apple Support Communities. We understand that the Calendar app is not working as expected. The following steps may be able to help correct this behavior:

• "If an app doesn’t quit as expected, or it’s unresponsive, you can force quit the app (you may lose unsaved changes). Choose Apple menu  > Force Quit, select the app in the dialog that appears, then click Force Quit.
• If the app quit, reopen it by clicking Reopen in the dialog that appears.
• Restart your Mac by choosing Apple menu > Restart.
• Make sure the app is compatible with your version of macOS.
• Open another file in the app. There may be a problem with the original file.
• Check for updates to your software.
• Disconnect any peripheral devices you just connected (for example, a printer). They may not be compatible with your version of macOS.
• Uninstall any app plug-ins or enhancements from other manufacturers.
• If you recently updated the app or macOS, a plug-in or enhancement you installed may no longer be compatible.
• Diagnose problems on Mac to learn whether hardware you installed, such as additional memory, is incompatible, improperly installed, or defective."

More details here: If an app freezes or quits unexpectedly on Mac

All the best.

Attempt to open Calendar while depressing and holding the shift key.