For one thing, he could have sworn that once or twice when he was reaching out to turn over a card that had been dealt to him, the card changed. It had changed = just like that – while he was picking it up.”
-- Narrator (chapter 1)

Importance: Lewis has just arrived at Jonathan's house and is playing poker with Jonathan and Florence. This is the first clue that Jonathan and Florence have magic powers, though Lewis never addresses the probability that they actually changed the cards to let him win.

Lewis soon found out that three weeks was not nearly enough time for exploring the town of New Zebedee and the house at 100 High Street. In three weeks, he barely got started."
-- Narrator (chapter 2)

Importance: Lewis arrives three weeks before school starts. He spends those weeks exploring Jonathan's house and the town. The fact that there are so many interesting things to see and do sets the stage...