The Lord's day mentioned by John was the Sabbath, the day on which Jehovah rested after the great work of creation, and which He blessed and sanctified because He had rested upon it. The Sabbath was as sacredly observed by John upon the Isle of Patmos as when he was among the people, preaching upon that day. By the barren rocks surrounding him, John was reminded of rocky Horeb, and how, when God spoke His law to the people there, He said, “Remember the sabbath day, to keep it holy” (Exodus 20:8). SL 74.2
The Son of God spoke to Moses from the mountaintop. God made the rocks His sanctuary. His temple was the everlasting hills. The Divine Legislator descended upon the rocky mountain to speak His law in the hearing of all the people, that they might be impressed by the grand and awful exhibition of His power and glory, and fear to transgress His commandments. God spoke His law amid thunders and lightnings and the thick cloud upon the top of the mountain, and His voice was as the voice of a trumpet exceeding loud. The law of Jehovah was unchangeable, and the tablets upon which He wrote that law were solid rock, signifying the immutability of His precepts. Rocky Horeb became a sacred place to all who loved and revered the law of God. SL 74.3