Wait I found it, just took asking in a different (less specific) way:
https://stackoverflow.com/questions/5734365/run-a-command-at-a-specific-time#5734420
EDIT: Here is how I'm testing, and it seems to work:
$ at now + 1 minute warning: commands will be executed using /bin/sh at> DISPLAY=:0.0 /usr/bin/vlc '/home/OrangeGull/Music/Avicii/Levels/Avicii - Levels.mp3' at> <EOT> job 33 at Mon Nov 14 19:07:00 2016
The trick is in DISPLAY=:0.0
You could make /var a separate writable partition
https://dietpi.com/phpbb/viewtopic.php?f=11&t=7595&hilit=Read+only+Fstab
https://github.com/MichaIng/DietPi/issues/3511
Verify your OS version in comparison to the above before you get crazy.