In a very limited-edition book titled About Centennial: Some Notes on the Novel, written by Michener in 1974, he says on the very last page (57):
"Its name comes from the fact that Colorado joined the Union in 1876, and is therefore known as the Centennial State."
It also happens that earlier in the book, he discusses being very depressed that the grand plans for a magnificent, intellectually sound Bicentennial celebration for the country, which he had been tasked to present to Congress for approval and funding in 1970, had gotten largely scuttled. On p. 27 he says:
Probe as I might, I could find no hope of salvaging any celebration on a national scale. Some of the individual states were conceiving imaginative small programs, but the grand design was dead, and it occurred to me that the best we could hope for would be for each citizen in our country to assume responsibility for his or her own centennial celebration. Each of us must affirm his own commitment to the future.
Once I visualized this, my responsibility became clear. I would use my knowledge of my nation and write about its spiritual condition as honestly as I could. My vehicle would be the western novel I had contemplated for so long.
So the name came from the nickname for Colorado, but the impetus for writing it came from the national bicentennial.
