It's not at all a guarantee that Galahad will be in the room during the day -- he and Claudius both have plenty to do, and he's been actively seeking out people to draw for his hagiographies -- but Magnus is lucky. This fast has has been harder than usual, perhaps because of the long winter season (Lent and Easter are very early this year, but not so early that Galahad can't tell the seasons here are shifted, elongated, like a ball of dough being rolled out into something great and thin), and Galahad is got up long enough to make sure Claudius had breakfast and then went back to bed.

He gets up when he hears the knocking -- only one person has the strength to pound on the door like that -- and goes to open it for Magnus.