A long, long time ago, there lived a Roman poet called Horace. (Actually, he was called Quintus Horatius Flaccus, but who's keeping track?). This fellow Horace wrote some poems, and in one of them, he told his readers to carpe diem—that is, to seize the day.
We're willing to bet you've heard that saying thrown around before: after all, it's the same basic concept that inspired Y.O.L.O. Seize the day (or night), Shmoopers, because for all we know, we might not get tomorrow!
Now, let's bring this back to Bellow.
Before settling on the title Seize the Day, Saul Bellow named earlier drafts of the novel "One of Those Days," "Here and Now—Here and Now," and "At the Foot of Mt. Serenity." The three later choices all reflect the carpe diem philosophy that Dr. Tamkin tries to peddle to Wilhelm. Just listen to Tamkin describe his counseling practice to his young apprentice:
The spiritual compensation is what I look for. Bringing people into the here-and-now. The real universe. That's the present moment. The past is no good to us. The future is full of anxiety. Only the present is real—the here-and-now. Seize the day. (4.71)
There is, of course, a level of irony to the novel's title. Tamkin's advice to Wilhelm isn't sound—in fact, it's really manipulative. By convincing Wilhelm to "seize the day," Tamkin gets his hands on the last of Wilhelm's savings. And, as Wilhelm's bad decisions have proved over and over again throughout his life: if seizing the day means moving forward without a good plan for the future, it's your own tough luck if you fail.