This reading is a portion of the Torah reading for Shavuot, which begins this year at sundown May 16.
On the third day, as morning dawned, there was thunder, and lightning, and a dense cloud upon the mountain, and a very loud blast of the horn; and all the people who were in the camp trembled. Moses led the people out of the camp toward God, and they took their places at the foot of the mountain.
Now Mount Sinai was all in smoke, for the Lord had come down upon it in fire; the smoke rose like the smoke of a kiln, and the whole mountain trembled violently. The blare of the horn grew louder and louder. As Moses spoke, God answered him in thunder. The Lord came down upon Mount Sinai, on the top of the mountain, and the Lord called Moses to the top of the mountain and Moses went up. The Lord said to Moses, "Go down, warn the people not to break through to the Lord to gaze, lest many of them perish. The priests also, who come near the Lord, must stay pure, lest the Lord break out against them." But Moses said to the Lord, "The people cannot come up to Mount Sinai, for You warned us saying, 'Set bounds about the mountain and sanctify it.'" So the Lord said to him, "Go down, and come back together with Aaron; but let not the priests or the people break through to come up to the Lord, lest He break out against them." And Moses went down to the people and spoke to them.