How the arrival and departure of Dill helps signify the passage of time in To Kill a Mockingbird?

Dill comes every summer, which helps signify the passage of time.

Dill is a friend of Scout and Jem who comes every summer to stay with his Aunt Rachel, and leaves at the end of the summer.  The children begin to count time by when Dill shows up.

Dill was from Meridian, Mississippi, was spending the summer with his aunt, Miss Rachel, and would be spending every summer in Maycomb from now on. His family was from...

Dill comes every summer, which helps signify the passage of time.

Dill is a friend of Scout and Jem who comes every summer to stay with his Aunt Rachel, and leaves at the end of the summer.  The children begin to count time by when Dill shows up.

Dill was from Meridian, Mississippi, was spending the summer with his aunt, Miss Rachel, and would be spending every summer in Maycomb from now on. His family was from Maycomb County originally… (Ch. 1)

Dill, Jem, and Scout are like the Three Musketeers.  They do everything together all summer long.  Their favorite pastimes are acting out books and trying to make Boo Radley come out.

Summer was our best season: it was sleeping on the back screened porch in cots, or trying to sleep in the treehouse; summer was everything good to eat; it was a thousand colors in a parched landscape; but most of all, summer was Dill. (Ch. 4)

Dill does not have a very good home life.  His mother remarries and Dill does not really like the new man.  He feels like his parents are not paying any attention to him, and runs away to Maycomb.  Dill is used to going to Maycomb every summer and looks forward to it just as much as Scout and Jem do.  They are his second family.

Since Dill is in Maycomb, he attends the trial proceedings with Scout and Jem.  Dill turns out to be just as sensitive as Jem.  He does not understand racism any more than Jem does and he has a violent physical reaction to the way that Tom Robinson is treated during the trial.

Dill is playful, creative, and loyal.  He is a good friend to Scout and Jem, and is an important character in the book.  The book's events are a coming of age journey for Scout and Jem, but for Dill as well.