The Sabbath of all sabbaths!

So, why did God rest on the 7th day, at the beginning? For this day, today, Holy Saturday. From Matins according to the Byzantine rite:

 Today Thou dost keep holy the seventh day, which Thou hast blessed of old by resting from Thy works. Thou bringest all things into being and Thou makest all things new, observing the sabbath rest, my savior, and restoring Thy strength.

Jesus Christ our God rests. It's the completion of the work of creation, which will be renewed tonight when he comes forth from the tomb, on the 8th day. After the resurrection, it's a whole new ball game.