Something really big must be happening in three days.
God said to Moses, "I will come to you in a thick cloud, in order that the people may hear when I speak with you and so trust you ever after." Then Moses reported the people's words to God, and God said to Moses, "Go to the people and warn them to stay pure today and tomorrow. Let them wash their clothes. Let them be ready for the third day, for on the third day God will come down, in the sight of all the people on Mount Sinai. You shall set bounds for the people round about, saying, 'Beware of going up the mountain or touching the border of it. Whoever touches the mountain shall be put to death: no hand shall touch him/her, but s/he shall be either stoned or shot; beast or human, s/he shall not live.' When the ram's horn sounds a long blast, they may go up on the mountain."