John Gill Commentary Exodus 19:14

"And Moses went down from the mount unto the people, and sanctified the people; and they washed their garments." — Exodus 19:14 (ASV)

And Moses went down from the mount unto the people
The same day that he went up, the fourth day of the month:

and sanctified the people ;
instructed them and ordered them what they should do for their sanctification, in order to their hearing the law from the mouth of the Lord:

and they washed their clothes ;
as the Lord had directed Moses to enjoin them, and as he had commanded them, (See Gill on Exodus 19:10).