Arbuckle behavioral studies 101
Jon takes pleasure in the small triumphs and joys of life, and that's something I like about him. His zero-to-sixty ebullition in one panel, is comedy technique that I have no name for besides "Why are you acting that way, if you know the turn?" In this case, why is Jon acting sad when recounting the story, if he knows that it's Wednesday?
Anyway, this is followed by an expression of contentment that I find particularly hilarious. It's not like the day of the week had any particular bearing on Jon's planned activities. Indeed, I assume he sits at the table happily reflecting on his triumph for several hours.
Garfield, meanwhile, thinks in boldface for the punchline, perhaps to be heard over the cacophony of the fourth wall shattering.
Synchronicty: This morning I thought it was Thursday. But it turned out to be Wednesday.