Charles Ellicott Commentary Exodus 19:11

"and be ready against the third day; for the third day Jehovah will come down in the sight of all the people upon mount Sinai." — Exodus 19:11 (ASV)

Against the third day. —There is no special “significance” in this mention of “the third day.” The important point is, that the purification was to continue through two entire days—one day not being sufficient. This taught the lesson that man’s defilement is, in the sight of God, very great.

The Lord will come down in the sight of all the people. —See the comment on Exodus 19:9.