Comentário sobre Êxodo 19:15

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).

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.

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).

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)

The woman might expel the seed. . . You might ask: Why was the Torah particular about impurity from seed, and not about impurity from zav, tzaraas or relations with a nidah? Tosafos in Shabbos 86a already answered: [In earlier times, someone impure from seed was forbidden to study Torah]. See there.

לשלשת ימים, according to the plain meaning of these words the meaning is: “in anticipation of the third day.” According to the plain meaning, Moses did not add an additional day of preparation (sanctification) as suggested by some scholars. Other examples of a similar formulation in the Bible as here are Genesis 42,17: ויאסוף אותם אל משמר שלשת ימים, “Joseph put them in jail for three days.” Or, in the verse following: ויאמר אליהם יוסף ביום השלישי, “Joseph said to them on the third day;” or Genesis 40,19: בעוד שלשת ימים, “within another three days;” and in the verse following: ויהי ביום השלישי, “it was on the third day;” or Exodus 19,16: ויהי ביום השלישי, “it was on the third day;” there are numerous other examples.