Exodus 19:11 in the King James Version
And be ready against the third day: for the third day the LORD will come down in the sight of all the people upon mount Sinai.
Context
10 The LORD said to Moses, “Go to the people, and sanctify them today and tomorrow, and let them wash their garments,
11 and be ready for the third day; for on the third day the LORD will come down in the sight of all the people on Mount Sinai.
12 You shall set bounds to the people all around, saying, ‘Be careful that you don’t go up onto the mountain, or touch its border. Whoever touches the mountain shall be surely put to death.