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."