Albert Barnes Commentary Exodus 19:10

"And Jehovah said unto Moses, Go unto the people, and sanctify them to-day and to-morrow, and let them wash their garments," — Exodus 19:10 (ASV)

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.