What is the significance of the title The Wednesday Wars?

The title of this book directly relates to events in the story. If I'm honest, the "war" part of the title fits better with the first half of the book.
The book begins with Holling Hoodhood narrating a little bit about himself, his family, and his school. Regarding the book's title, his narration about the school is most important. Holling attends a school in which all of the students are either Catholic or Jewish. Holling doesn't fit with either group because his family is Presbyterian. During most of the week, Holling's religious affiliation has almost zero impact on the story; however, Wednesday afternoons is a different story. At 1:45 half of his class goes to Hebrew School and the other half goes to Catechism. Holling is the only student that is left behind, and the school needs to figure out something to do with him.
The solution is to send him to Mrs. Baker's classroom for a study hall of sorts. Mrs. Baker is not happy about the situation, and Holling knows it. In fact, Mrs. Baker even tries to get rid of Holling by having him go to a different class and retake 6th grade math. That doesn't work out, so Holling and Mrs. Baker get to spend Wednesday afternoons with each other. For the first month, Mrs. Baker has Holling do dirty, manual labor tasks like cleaning desks, chalkboards, and erasers. Mrs. Baker and Holling hardly talk, and Holling truly believes that Mrs. Baker hates him.
The manual labor lasts about a month, and then Mrs. Baker changes her mind about what to do with Holling. In October, Mrs. Baker tells Holling that on Wednesday afternoons, they will be reading Shakespeare together. Holling is appalled. He believes that Mrs. Baker has now found a new way to punish him way beyond cleaning erasers.

Reading Shakespeare. Of all the strategies Mrs. Baker could come up with, this must be the worst. Teachers bring up Shakespeare only to bore students to death. And I was going to be bored to death for eight months. No human being could stand it.

Holling even suggests that he should just stick to cleaning the classroom. The Shakespeare reading starts off rocky, but it slowly improves for Holling, and he learns to like it a lot. Holling and Mrs. Baker slowly begin to start getting along. They each learn a lot about the other, and a deep mutual respect is born. They find themselves helping each other in ways that show a much deeper relationship than a standard teacher and student relationship.