Sanctify them - The injunction involves bodily purification and undoubtedly also spiritual preparation. Compare Hebrews 10:22. The washing of the clothes was an outward symbol well understood in all nations.
Related Commentaries of Exodus 19:10
Exodus 19:10
10 And the LORD said unto Moses, Go unto the people, and sanctify them to day and to morrow, and let them wash their clothes,