I went to sleep with gum in my mouth and now there's gum in my hair and when I got out of bed this morning I tripped on the skateboard by mistake […] (1)
This is the start of it all. The funny thing is, the very first thing that "happens to" Alexander is a direct result of his own irresponsibility: going to bed with gum in his mouth, not to mention all the junk he's left on the floor. But don't say that to Alexander. It'll make things much worse.
[…] but in my breakfast cereal box all I found was breakfast cereal
In the context of the bad day, even totally ordinary things are no good and very bad. The cereal box toy is special—a seemingly once-in-a-lifetime (or just once-in-a-week-or-so) treat. So even though the odds were not in his favor, Alexander feels totally ripped off.
I think I'll move to Australia (3)
The ultimate escape from dissatisfied life. Just bring your sunscreen.
But it'll probably just get in your eyes.
In the car pool Mrs. Gibson let Becky have a seat by the window. Audrey and Elliott got seats by the window too (4).
Notice the language Viorst uses. The adult "gives" a seat to Audrey, and Elliott also receives something. But Alexander? Nope. Nada. Nothing. Zilch.
Oh, the humanity!
Guess whose mother forgot to put in dessert? (10)
On the way downstairs the elevator door closed on my foot and while we were waiting for my mom to go get the car Anthony made me fall where it was muddy and then when I started crying because of the mud Nick said I was a crybaby and while I was punching Nick for saying crybaby my mom came back with the car and scolded me for being muddy and fighting. (15-16)
This is pretty much the pinnacle of Alexander's suffering. The run-on sentence is more than you could say in a single breath, emphasizing just how exhausting all this injustice really is.