"We'd be best pals if they didn't keep trying to do me in." -- Harry Potter
Harry tries one of the Half-Blood Prince’s spells while reading in bed and levitates Ron by accident. He argues with Hermione over the danger of the Prince, and gets word of his next lesson with Dumbledore. The trio then walks to Hogsmeade, where Slughorn corners Harry in Honeydukes and Harry sees Mundungus with things stolen from Grimmauld Place. On the way home, an Imperiused Katie Bell touches a cursed necklace and is gravely injured; Harry then suspects Draco and tells McGonagall this, though Ron and Hermione don’t believe him.
Calendar and Dates
The entire chapter takes place in one day, which we know is "halfway through October," and which is almost certainly a Saturday like all Hogsmeade days have been. The date is most likely either 12 or 19 October, but as the dates get even more difficult to determine in the next two chapters, there's no way to determine for sure.
He did not usually lie in bed reading his textbooks; that sort of behavior, as Ron rightly said, was indecent in anybody except Hermione, who was simply weird that way.