This is an expression I encountered in Wandering Shadow. It means to be absent without reason. This comes from the traditional Chinese calender (the types that you tear off a page everyday), as a green day/date detones a working day while a red day/date means a Holiday/weekend.Turning the calenders from Green to Red
Now while I have n problems with this, I'm wondering if its in the original Japanese text. Since I'm translating from a chinese fan source, it does make me wonder if its in the original japanese text / the official Kadokawa text.
Can someone please clarify?