I recently find my solution to this on desktop Linux:

+ The machine has "Do not Disturb" on, forever.

+ My todo is a never ending text file. Every appointments are laid out on previous day with the following format:

  *- 09:00 do something*

+ There is a script that runs periodically that checks updates of my todo file, and creates systemd timers that will trigger "critical" level notifications that are not blocked by "Do not Disturb".

  *systemd-run --user --on-calendar="$trigger_time" --unit=$unit_name sh -c 'paplay "$1" && notify-send -u critical Appointment "$2"' _ "$SOUND" "$title"*