According to Exodus 19, the Israelites were told to prepare for three days before God descended on Mount Sinai.
After leaving Egypt, the Israelites reached the wilderness of Sinai in the third month. They camped opposite the mountain, where Moses went up to receive God’s instructions. The people were told to consecrate themselves, wash their clothes, and be ready on the third day.
Exodus describes strict boundaries around the mountain because the people were not permitted to approach it freely. On the third day, thunder, lightning, a thick cloud, and a very loud trumpet blast appeared. The mountain shook, and smoke rose from it because God descended in fire.
This three-day preparation is distinct from other time periods in the Exodus narrative. Moses later spent forty days and forty nights on the mountain, while Exodus 24 says the cloud covered Sinai for six days before God called to him on the seventh day. Those separate numbers are common sources of confusion.