In the old creation the week began with work and ended with Sabbath rest. The resurrection week begins with the first day -- first rest, then labor.
So we must first cease from our own works as God did from His, and enter into His rest, and then we will work, with rested hearts, His works with effectual power.
But why "labor to enter into rest"? See that ship -- how restfully she sails over the waters, her sails swelling with the gale; and borne without an effort! And yet, look at that man at the helm. See how firmly he holds the rudder, bearing against the wind, and holding her steady to her position. Let him for a moment relax his steady hold and the ship will fall listlessly along the wind. The sails will flap, the waves will toss the vessel at their will, and all rest and power will have gone. It is the fixed helm that brings the steadying power of the wind. And so He has said, "Thou wilt keep him in perfect peace, whose mind is stayed on Thee, because he trusteth in Thee." The steady will and stayed heart are ours. The keeping is the Lord's. So let us labor to enter and abide in His rest.