What does "li'Sheloshes Yamim" mean?
Rashi #2 and Targum Yonasan (in Pasuk 16) 4 : It means on the third day - Shabbos, the sixth of Sivan.
According to Rebbi Yossi, Shabbos, 87a), who maintains that Moshe added a day of his own accord (Rashi).
Sifsei Chachamim: Since Moshe changed from "ba'Yom ha'Shelishi" to "Sheloshes Yamim".
Rashi: According to the Chachamim of Rebbi Yossi (Shabbos, 87a).
What does "Al Tigshu el Ishah imply? What are its ramifications?
Rashi: It means that they were to abstain from intimacy during the three days, lest the woman exude the Zera, which would render her Tamei again. 1
Moshav Zekenim: Even though a woman can clean herself, and then she can immerse and eat Terumah at night (Yevamos 69b), permitted she would not clean herself well.
Why specifically three days?
Rashi: Because Zera that a woman exudes after three days is not Metamei. 1
Since it is not fit to germinate (Rashi). See also Ba'al ha'Turim on Pasuk 16.
Rashi writes that they abstained from intimacy during the three days, lest the woman exude the Zera, and become Tamei. According to Rabanan, Torah was given to on the third day from the command. They abstained only two days!
Moshav Zekenim: If she emits, she can immerse, and she is Tehorah for Divrei Torah. A Ba'al Keri need not wait until night. Even so, it was proper to abstain for three days; part of the third day is like the entire day.
Rashi writes that they abstained from intimacy during the three days, lest the woman exude the Zera, and become Tamei. Before the Torah was given, Yisrael did not receive Tum'ah!
Riva: The Torah was adamant about what would be Tamei after the Torah was given. 1
Riva: Even though the Torah was not adamant about others who would be Tamei after Matan Torah, e.g. a Zav, Metzora or Bo'el Nidah, one who is Tamei due to Bi'ah is different, for it comes from lightneadedness, and Torah must be amidst fear and awe.
Moshe decided to separate from his wife due to a Kal v'Chomer, and Hashem agreed (Shabbos 87a). Bnei Yisrael spoke with Hashem once, and they needed to separate. Hashem can speak to me any time, all the more so I must separate. It was more proper for Yisrael to separate, for Hashem fixed a time when Vadai He will speak to them!
Moshav Zekenim: It should have sufficed for Yisrael to abstain for one day. They abstained for three days, due to Safek lest a woman emit Zera. All the more so Moshe should abstain, due to Safek lest Hashem speak to him while he is having relations!
This is why it says that Moshe decided. A Kal v'Chomer is mid'Oraisa! This was not an absolute Kal v'Chomer. 1 (PF)
Riva: It was not an absolute Kal v'Chomer, due to Aseres ha'Dibros. (I.e. Aseres ha'Dibros are more severe than Stam Dibur of Hashem. Alternatively, he means that Hashem had fixed a time for Aseres ha'Dibros