Welcome to LQ!
In the future, try to give as much information as you can, particularly in this case, which Linux distribution you are using and which desktop environment you use (GNOME, KDE, Unity, Mate, etc.).
If you use Google calendar, you can sync your calendar with a desktop app like Thunderbird, Kmail, or others and set an appointment and tell it to remind you with a popup on the desktop or send you an email. If you want a standalone program you can try Alarm-Clock
, or Reminder-ng
. These apps may be in your distributions repositories as well. I'm sure there are others too.
Hope this helps, have fun!