Turn off calendar notifications - Ubuntu 20.04

If you added online accounts, go to settings > Applications > calendar, then turn off notifications

Or you can go to settings > online accounts > google account the calendar option

You can suppress the ugly pop-up window – but keep the less obtrusive Gnome Shell notification – by setting /org/gnome/evolution-data-server/calendar/notify-with-tray in dconf.

I discovered this from a comment in a related evolution-data-server bug report (GNOME/evolution-data-server#60).

Option 1: Use dconf Editor

Install dconf Editor if it's not already installed:

sudo apt install dconf-editor

Launch dconf Editor:

Navigate to /org/gnome/evolution-data-server/calendar and set notify-with-tray to "on":

Option 2: Use gsettings

Alternatively, a quicker way to turn this setting on is to issue following command in the terminal:

gsettings set org.gnome.evolution-data-server.calendar notify-with-tray true