Writers - Where would I put this?

In my novel Asylum I want to put in a bonus chapter describing Todd's first days at Asharet. I'm unsure, however, on where exactly to put it; I'm not sure about putting it at the end because by then they are out of Asharet and it wouldn't be relevant, but I don't know where to put it. Should I have it as a interlude chapter somewhere in the middle? Or can I have some more suggestions? :3