On exactly the third-month anniversary of the Israelites' leaving the land of Egypt, they came into the Sinai desert.
References for Exodus 19:1
They traveled from Rephidim, came into the Sinai desert, and set up camp there. Israel camped there in front of the mountain
while Moses went up to God. The LORD called to him from the mountain, "This is what you should say to Jacob's household and declare to the Israelites:
You saw what I did to the Egyptians, and how I lifted you up on eagles' wings and brought you to me.
So now, if you faithfully obey me and stay true to my covenant, you will be my most precious possession out of all the peoples, since the whole earth belongs to me.
You will be a kingdom of priests for me and a holy nation. These are the words you should say to the Israelites."
So Moses came down, called together the people's elders, and set before them all these words that the LORD had commanded him.
The people all responded with one voice: "Everything that the LORD has said we will do." Moses reported to the LORD what the people said.
Then the LORD said to Moses, "I'm about to come to you in a thick cloud in order that the people will hear me talking with you so that they will always trust you." Moses told the LORD what the people said,
References for Exodus 19:9
and the LORD said to Moses: "Go to the people and take today and tomorrow to make them holy. Have them wash their clothes.
Be ready for the third day, because on the third day the LORD will come down on Mount Sinai for all the people to see.
Set up a fence for the people all around and tell them, ‘Be careful not to go up the mountain or to touch any part of it.' Anyone who even touches the mountain must be put to death.
No one should touch anyone who has touched it, or they must be either stoned to death or shot with arrows. Whether an animal or a human being, they must not be allowed to live. Only when the ram's horn sounds may they go up on the mountain."
So Moses went down the mountain to the people. He made sure the people were holy and that they washed their clothes.
He told the men, "Prepare yourselves for three days. Don't go near a woman."
When morning dawned on the third day, there was thunder, lightning, and a thick cloud on the mountain, and a very loud blast of a horn. All the people in the camp shook with fear.
Moses brought the people out of the camp to meet God, and they took their place at the foot of the mountain.
Mount Sinai was all in smoke because the LORD had come down on it with lightning. The smoke went up like the smoke of a hot furnace, while the whole mountain shook violently.
The blasts of the horn grew louder and louder. Moses would speak, and God would answer him with thunder.
The LORD came down on Mount Sinai to the top of the mountain. The LORD called Moses to come up to the top of the mountain, and Moses went up.
The LORD said to Moses, "Go down and warn the people not to break through to try to see the LORD, or many of them will fall dead.
Even the priests who come near to the LORD must keep themselves holy, or the LORD will break loose against them."
Moses said to the LORD, "The people aren't allowed to come up on Mount Sinai because you warned us and said, ‘Set up a fence around the mountain to keep it holy.'"
The LORD said to him, "Go down, and bring Aaron back up with you. But the priests and the people must not break through and come up to the LORD. Otherwise, the LORD will break loose against them."
So Moses went down to the people and told them.