It's a week into summer, and Skye is bored. It might be a case of cabin fever, but she needs more than just sitting outside and reading about the holographic universe.
Jeffrey writes her a letter. It's summer and he doesn't have anything else to do besides eating and guitar playing, so he does it for the heck of it. Sure, he could call her or text her, but he thinks everyone could use a letter addressed to them.
He ends up writing letters to the rest of her family, even Hound, and sends them first class at the post office.
Three days later, he gets an invitation to visit the Penderwick household, signed by everyone, even Hound.
He accepts.