Just create a new event with whatever you need to be reminded of. Set alarm to message (or even message with sound), and make sure it repeats every day.
You can also choose to run a script instead of showing an alarm. The alarm will look like this:
Using crontab and AppleScript
If you're more into doing command line stuff, you can also open a Terminal, and enter
EDITOR=nano crontab -e
A simple text editor will open. Paste the following:
0 13 * * * osascript -e "tell application \"Finder\" to display dialog \"Do your stuff\"" > /dev/null 2&>1
This will launch a simple AppleScript command that displays a Finder dialog with the message "Do your stuff". Feel free to change this.
You can also change the date – in this case, 0, 13 and the stars mean "13h and 0 minutes", at every day, every week, every month. Check this Crontab Generator for the syntax.
Press Ctrl-O, then Enter to save it, and then Ctrl-X to exit the editor.
To disable this alert again, just call the
crontab -e command, and delete the line.