I'm designing an app which will allow a user to both schedule playlists at different times of the day and override that schedule to play a playlist. How do I communicate to the user that playing a playlist will override their current schedule and how do I allow them to switch back to schedule mode?
I would give them a choice when they try to start a playlist that will interfere with their schedule.
I might be tempted to show which playlist(s) are affected, depending on how much information you want them to have.
As for "how do I allow them to switch back to schedule mode?", that's a little difficult to answer without seeing the rest of the interface. I do certainly think it's important that, whether the current playlist is scheduled or started manually, the user can tell the difference.
I guess a playlist is a fixed-length event in a calendar-like something, even if more precise than calendar events usually are.
done using the fullcalendar jQuery plugin
I don't see why you need modality here, it looks more like a list-detail issue to me.