From the beginning of the novel, Silko makes it apparent that the concept of "ceremony" is a pretty big deal. It's an idea that's rooted in the oral tradition of Laguna Pueblo culture.
The novel opens with three short poems, and two of them contain the word ceremony. What are the chances? The third poem tells us that a ceremony is a kind of cure ("The only cure I know is a good ceremony"), while the second poem connects ceremonies to stories. These stories are also a kind of cure; think of them as preventive medicine.
It's not really clear where stories end and ceremonies begin. A lot of stories are ceremonies in their own ways. Remember the drinking stories Emo tells his buddies about macking on white women? It doesn't seem like a very ceremonial subject, but it does become a ritual.
And the "official" Laguna and Navajo ceremonies in the novel—the ones performed by medicine men old Ku'oosh and Betonie—all contain stories. Some of the ceremonies even contain stories within stories: for example, old Betonie's story about the invention of white people features a storytelling competition among a group of witches. A good portion of his story is actually the retelling of a story told by one of the witches. Meta.
So the ideas of ceremony and story are impossible to separate. In Tayo's case, completing his ceremony means completing his story.
Likewise, his entire story is part of his ceremony, not just the parts where he's performing rituals or communing with nature. The ugly parts of the story—the racism, alcoholism, and atomic warfare—might not seem very ceremonial. After all, these are very modern problems. But as Betonie explains, both ceremonies and stories have to evolve to keep up with current events.
When we realize that stories are ceremonies, it becomes clear that this novel itself is a ceremony—hence the title. Ceremony isn't a novel about a ceremony; it is a ceremony.