What was Hashem's response to their request?
Rashi: Since they were now destined to hear the Aseres ha'Dibros directly from Hashem, they would require a process of purification that would last for three days - from the fourth to the sixth of Sivan, 1 as the Pasuk goes on to explain.
On the fifth of Sivan, Moshe built a Mizbe'ach and twelve Matzeivos (monuments) at the foot of the mountain, as the Pasuk describes at the end of Mishpatim (Rashi).
What is the meaning of "v'Kidashtem"?
Rashi, Ramban #1 (citing Onkelos) and Seforno: It means "Prepare yourselves". 1
Ramban #2 (citing Ibn Ezra): It means to immerse in water. 2
Ramban #3: It means "Sanctify yourselves" - by keeping away from one's wives and from all kinds of Tum'ah. 3
See Ramban's objection to this explanation.
What did the preparation entail?
Rashi and Seforno: It entailed separating from their wives (abstaining from intimacy). 1
Why did they need to wash their clothes?
Mechilta: Whenever the Pasuk mentions Kibus Begadim, it means that there was Tevilah as well. 1
Where (and when) was this Pasuk said to Moshe?