For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Chapter 1 – Chapter 2
Technically, the "first act" of Seize the Day happens offstage. Four days before the morning when the novel begins, Tommy Wilhelm gives Dr. Tamkin a power of attorney over the last of his measly savings, and lets him invest it all in the commodities market (in lard). Yeah, not the best idea…we wouldn't exactly describe lard as a "hot commodity." By the time the novel opens, the consequences of Wilhelm's actions are already underway.
That said, it isn't until the very end of Chapter 2 that we the readers finally learn what Wilhelm has done, and that's what makes this moment such a crucial turning point in the novel.
Act II
Chapter 3 – Chapter 6
Not a lot of time passes in the novel's "second act": no more than four or five hours, tops. But in the time it takes to finish his first breakfast with his father and his second breakfast with Tamkin, visit the brokerage office, have lunch with Tamkin (how much food does this guy need to eat?), and to return to the brokerage office once more, Wilhelm's world comes crashing down around him. By early afternoon, all of his worldly assets are gone, and the wolves (aka his wife and his building manager) are howling at the door.
Act III
Chapter 7
In the novel's final chapter, Wilhelm makes a last-ditch attempt to beg his father for help and find Dr. Tamkin. When neither plan works out, our poor protagonist finds himself out in the street, being herded into a funeral parlor along with a crowd of mourners. Whatever Wilhelm chooses to do in the days to come, the novel's third and final act ends with him sobbing inconsolably in a corner, as the rest of his fellow creatures look on.