How to fix random disappearance of Calendar notifications on M1 iMac running macOS Sequoia?

After some research, here's what I found:

(extracted from the built-in Apple app, Tips. You can also find helpful troubleshooting articles in there.)

If you set an alert to occur before a calendar event but aren’t receiving the alert in Notification Center,

• Make sure alerts aren’t turned off in Calendar settings. Go to the Calendar app on your Mac, choose Calendar > Settings, click Alerts, then select “Show shared calendar messages in Notification Center” and “Show invitation messages in Notification Center.”
• Make sure alerts aren’t turned off in Notifications settings. Choose Apple menu  > System Settings, then click Notifications in the sidebar (you may need to scroll down). Go to Application Notifications, click Calendar, then make sure “Allow notifications” is turned on.
• Make sure a Focus isn’t active when you want to receive alerts. See Turn a Focus on or off.
• Make sure event alert times are set correctly. Choose Calendar > Settings, click Alerts, then choose an alert time other than None for Events, All Day Events, or Birthdays.
• Make sure you set up the alert properly. See Set alerts for an event.
• If the alert is set to send an email, make sure your email address is correct in your card in Contacts. See Update contact information.

I am sooo sorry that your calendar is not working! It is a great free tool for people who keeps forgetting things (like me), but I guess my Mac skills are my weak spot 🫥

Best of luck to you 🍀

Go to Settings > Notifications > Calendar, and check if your notifications for calendar is on.

If you are using a M1 chip, there might be some bugs after long usage. Upgrading your Mac could help. I recommend doing that, based on my experience. I am running macOS Tahoe 26.2 along with the M4 chip, with liquid glass features and stronger imagery. You can upgrade your Mac in Settings > General > Software Updates.

Here's a chart that compares the Tahoe and Sequoia system: (macobserver.com)

Hope I helped 🍀👍