Okay, now we're back in the present (you know, the whole curious incident present). Christopher takes the bus to school the next morning.
They pass four red cars in a row, which means it is a "Good Day," so Christopher decides not to be sad about Wellington anymore.
Our narrator then explains the difference between a "Good Day" and a "Quite Good Day" and a "Super Good Day" (which are, of course, all awesome); and a "Black Day" ("a day when I don't speak to anyone and sit on my own reading books and don't eat my lunch and Take No Risks" [47.2]).
Long story short: the quality of the day depends on which cars he sees while he's riding the bus. Red cars are good; yellow cars are bad.
The school psychologist once told him that it's random and illogical being so affected by cars like that (and particularly strange because Christopher is otherwise a very logical guy). But Christopher argues that it's no more illogical to be affected by red and yellow cars than it is to be affected by the weather – to be happy if it's sunny and sad if it's cloudy. Touché!
Christopher then offers up some more examples of so-called "normal" behavior being illogical, and his own "unusual" behavior being really quite normal.
He recalls the school psychologist asking some other things, like whether it makes him feel safe to always have things in a nice order (answer: yes) and whether he doesn't like it when things change (answer: he wouldn't mind changing if he were changing into an astronaut).
Christopher says he probably won't become an astronaut, but he's definitely going to go to university (as they call it in the UK) to study Mathematics or Physics.
After that little digression, we hear more about his Good Day. Because it's a Good Day, he decides to try to figure out who killed Wellington, "because a Good Day is a day for projects and planning things" (47.15). Naturally.
Siobhan (his teacher) says that it's is a story-writing day, so maybe he should write a story about the night he found Wellington.
"And that is when I started writing this" (47.17).