11 and be ready by the third day, because on that day the LORD will come down on Mount Sinai in the sight of all the people.

Read Exodus 19:11 Using Other Translations

And be ready against the third day: for the third day the LORD will come down in the sight of all the people upon mount Sinai.
and be ready for the third day. For on the third day the LORD will come down on Mount Sinai in the sight of all the people.
Be sure they are ready on the third day, for on that day the LORD will come down on Mount Sinai as all the people watch.