Definitely start Monday
“Can’t be helped I suppose. I did rather spring it on you. We’ve been running a rota system for the cover since, since the other, other… well. Nothing for it I suppose. My turn tonight.” He leans over the desk and proffers his hand. “Welcome aboard, and see you Monday night, ten o’clock. Bring a flask and something to eat, and something to keep you occupied. If I’ve learnt anything from my stints, is that it’s really very boring.”
You shake his hand again, and leave his office, feeling slightly dazed. You have a job.