Lots of great things happen in those first 5 chapters of The Wednesday Wars. Many of them are important too, so you could pick just about any and defend why it's important.
I think a very important event is the fact that Holling must spend his Wednesday afternoons with Mrs. Baker. He's not Jewish, and he is not Catholic. That means he doesn't go to either group's special Wednesday afternoon religious studies. The fact that Holling is forced to spend time with Mrs. Baker is probably the single most important event of the entire book. If Holling never spends any time with Mrs. Baker, he never gets to play catch with the Yankees. He never gets to learn to love Shakespeare either. She is crucial to the success of his school year and his relationships with many of his closest friends.
Another event that you could pick is the event where Holling sidesteps the oncoming older brother of Doug Swieteck. Holling was invited to play in a pick up soccer game, and Holling became the target of Doug's older brother. Holling deftly got out of the way of the charging bully, tripped him in the process, and caused the bully to hit his head on the goal post. That single event kicked off the continuing battle between Holling and Doug's older brother.