According to the Christian Bible, true believers are commanded to remember the Sabbath Day to keep it holy... by doing nothing but praying. Then Sunday was deemed to be more convenient than the original Saturday (for practical Christians) and seemed a better day to which this remembering should be attached.
Convenience, you see, is paramount in the human psyche.
Whichever day it is doesn't matter to me because I don't believe in such childish mythology anyway, nor in an immaculately conceived, divine being called Jesus.
But Sunday being a day of rest is, to Americans, a recognized and widely accepted social convention, so I would probably be stupid not to observe it.
I'm gonna rest today.