The Sabbath Day represented the eternal spiritual rest that comes through faith in Christ (Hebrews 4:11).
But it also represented something else — namely, the means by which Jesus won that rest for us.
For the Sabbath day (Saturday) is the one full day that Jesus’ body lay in the grave. He died on Friday and rose on Sunday, but on Saturday there was no activity. His body lay lifeless in the tomb all day.
That’s one of the core meanings of the Old Testament Sabbath, and how Christ fulfills it. The Sabbath day represented the day Jesus’ body would lay lifeless in the tomb, to be raised the next day for our eternal salvation and rest in him.