Sinai: So Called Because It's a Sign, I ThinkBy Rabbi Jack Abramowitz
When the Jews heard G-d’s proposition, they agreed unanimously and unconditionally. That being the case, G-d gave them three days to prepare themselves. They were to purify themselves and even immerse their clothing in a mikvah (ritual bath). On the third day, G-d would manifest His Presence in a cloud on Mt. Sinai and all the Jews would hear Him speak to Moshe.
A perimeter was set around the mountain because any person or animal who touched it during the experience would die; a long shofar blast would sound the “all clear.”
The people purified themselves and abstained from marital relations for three days in anticipation of the encounter. On the third day, there was lightning and thunder with a heavy cloud on Mt. Sinai. The people trembled from the spectacle, but Moshe led them to the mountain. There, G-d’s Presence manifested in fire and the mountain was smoking like a furnace. The mountain shook and the sound of a shofar grew increasingly louder. Moshe spoke and G-d replied in an audible voice.