Шмот 19:15 Комментарий: Rashi, Chizkuni & Siftei Chakhamim

היו נכנים לשלשת ימים means be ready by the end of three days, that is, by the fourth day — for Moses added a day of his own accord to the time appointed by God. This is the opinion of Rabbi José. But according to the opinion of him who says that the Ten Commandments were given on the sixth of the month Moses did not add anything, and the words לשלשת ימים do not signify after three days but mean the same as ליום השלישי (v. 11), i. e. by three days (cf. Shabbat 86b-87a).

ויאמר אל העם, “he said to the people;” on the same day when he issued the instructions he told the people to be ready for the needs of what was going to occur at the end of this period of consecutive three days of sanctifying themselves, (by not having marital relations).

By the end of three days. . . [Rashi knows this] because it is not written ליום השלישי , as Hashem commanded Moshe — but rather לשלושת ימים , implying when three days have already passed.

אל תגשו אל אשה DO NOT GO NEAR A WOMAN during the whole of these three days, this was in order that the women may immerse themselves on the third day and be pure to receive the Torah. If they have relations within the three days, the woman could [involuntarily] expel semen after her immersion and become unclean again. After three days have elapsed, however, the semen has already become putrid and is no longer capable of fertilization, so it is pure from contaminating the woman who expels it. (cf. Shabbat 86a)