And be ready against the third day
Not the third day of the month, but the third day from hence, this being the fourth, and the morrow the fifth, and the third day, the day following that, the sixth, on which day it is generally agreed by the Jews that the law was given, (See Gill on Exodus 19:16)
for the third day the Lord will come down in the sight of all the
people upon Mount Sinai;
which must be understood, consistent with his omnipresence, and is only expressive of some visible display of his power, and of some sensible token of his presence to the people; he was now upon it in the pillar of cloud, but then he would appear in another manner, and descend in a thick cloud and fire, which all the people would see, though they could not see the similitude of anything in it.