An exception to my all-things-in-Emacs rule is my calendar. I do keep an Org mode agenda for things like projects and tasks to be done but for doctor’s appointments and the like I use the the macOS calendar app—actually Fantastical but that’s a detail. I do that because it makes my calendar easily available on all my devices. When I’m at, say, the doctor’s and am making an appointment for my next physical, it’s nice to be able to bring my calendar up on my iPhone so I can see what days and times are free.
Not everyone does things like me of course. Lots of folks keep their calendar in Org mode and for them there is a need for alerts of upcoming events in the agenda. Happily, Artem Khramov has met that need with org-wild-notifier.el. Take a look at the README to see the what sort of alerting it can do.
If you use Org for your calendar and you like to get alerts a few minutes before meetings or appointments, you should take a look at org-wild-notifier. It a reasonably light-weight solution that will probably serve your needs.