I guess Mom was pretty proud of herself for making me write in that journal last year, because now she went and bought me another one.
-- Greg (September paragraph 1)

Importance: This shows how Greg has continued to write in his journal, and despite his protestations, he actually enjoys it. His mother knows this and is encouraging it.

Our family didn't go anywhere or do anything fun, and that's Dad's fault."
-- Greg (September paragraph 4)

Importance: This indicates Greg's immaturity and lack of understanding about things. He thinks that they haven't gone anywhere because his parents have no interest, rather than considering that his parents couldn't get time off from work, or that they didn't have the money to go.

Then I remembered: I still had the Cheese Touch from last year."
-- Greg (September paragraph 30)

Importance: This refers back to book one where Greg touched the cursed piece of cheese. Once he "hands" off the "touch" to another student, he's relieved of the curse...