Why do Christians observe the Sabbath on Sunday?

While Jesus lay in the tomb, the heart of time skipped a beat.

Under natural circumstances, the going down and rising of the sun determines the ending and beginning of a day. But a supernatural event overshadows the natural, and the Divine Will subsumes the cycles of creation. Those who are of the Spirit know that the Light of the World is Jesus Christ, and His going down occurred the afternoon of the sixth day and His rising determined the start of the next day, the new Sabbath, the new seventh day. So what was formerly the first day of the week became the last, the Sabbath, which is the Hebrew word for seventh.