I think it was on the third day
This is a religious allusion to the resurrection of Jesus Christ. With the arrival of Henry Gatz, we have a sort of “resurrection” of James Gatz, the real boy with the real dreams of self-improvement and success, unfettered by the “foul dust that floated in the wake” of those dreams, the unreal dreams of materialism and luxury.
This stands in opposition to the language Fitzgerald uses in Chapter 6: “[Jay Gatsby] was a son of God… and he must be about His Father’s business”. In this case, Gatsby’s “father” is the American Dream of wealth and power. But James Gatz’s dream, as indicated by the passage a little later with his schedule written in Hopalong Cassidy book, was simply to make himself into, in Nick’s words “the well-rounded man.”
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