I tried to post to stackoverflow but I'm not sure it took, so I'm trying here as well:
I found this post which is getting quite old, and he says it has been implemented into Emacs but I can't see it anywhere. It's based on Orgs ability to mark weekends in different color.
Does anyone know where I start getting something like this working? Programming is not my strong suit, but I can slowly wrap my head around some bits of code.