What is "Ki Sheishes Yamim ... " referring to?
Seforno: It teaches us that the objective of Shabbos is to emulate Hashem and to be like Him - by reflecting, studying and in one's actions aimed at doing His will.
Oznayim la'Torah: It refers to "Zachor es Yom ha'Shabbos", in that by keeping Shabbos, one will remember that Hashem created the world in six days and will desist from idolatry. 1
See Oznayim la'Torah:
Why does the Torah omit the 'Beis' in the word "Sheishes"?
Ramban #1 (according to Kabalah): When the Pasuk writes "Ki Sheishes Yamim Asah Hashem", it means that Hashem created time, and on the seventh day (when time already existed) He rested. 1
Ramban #2 (according to Kabalah) 2 : The six days of the Creation represent the six thousand years that the world was destined to exist; 3 whereas the seventh represents the seventh thousand, when Mashi'ach will come.
Which had not existed prior to the Creation
See Rav Chavel's notes on the Ramban, who cites both interpretations of the Ramban's explanation.
This explanation is based on the premise that one day of ours is equal to a thousand years of Hashem (See Rashi in Tehilim 90:4).
What does "Vayanach ba'Yom ha'Shevi'i" mean vis-a-vis Hashem?
Rashi: Hashem used the term 'rest' regarding Himself for us to learn a Kal va'Chomer 1 - Man, who toils and becomes weary, should certainly 'rest' (cease working) on Shabbos.
Rosh: It means 'He ceased'. 2
Though the concept of 'Menuchah' is not applicable to Hashem.
Like Targum Yonason on Shmuel 1, 25:9.
What blessing did Hashem confer upon the Shabbos and how did He sanctify it?
Rashi and Rashbam: Hashem blessed it by sending a double portion of Manna on Erev Shabbos, 1 and sanctified it by not sending it on Shabbos. 2
Ramban #1: Hashem commanded us to remember it by blessing and honoring Him; and to sanctify it by desisting from working on it.
Ramban #2 (citing Ibn Ezra): He blessed it and sanctified it by providing a special Nefesh of additional wisdom.
Seforno: Hashem blessed it with an extra Nefesh and sanctified it inasmuch as everything that one does on Shabbos is in the Name of Hashem. 3
Refer also to 16:22:1:1*.
When Shabbos arrived, everything was in place, so that Shabbos was blessed with everything that was good. That is why one is able to sanctify it by resting on it as testimony that Hashem created the world in six days and 'rested' on the seventh (Rashbam).
Refer to 20:10:1:1*.
How will we reconcile the current Pasuk, which connects Shabbos with the Creation, with the Pasuk in Va'eschanan Devarim, 5:15, which connects it with Yetzi'as Mitzrayim?
Moshav Zekenim: Really, it is due to the creation. Only in case one thinks that he loses out through resting on Shabbos, the Torah writes in Devarim that we were slaves who never worked for themselves, and that how much more should we desist on Shabbos and Yom Tov for the sake of Hashem.
Commentaries: Because whereas here, with reference to their stay in the desert, where they did not work all week, the emphasis was on Kidush and the positive aspects of Shabbos ("Zachor"), in Va'eschanan, it was said in Arvos Mo'av, where they would begin wotking all week, from which they were being commanded to desist ("Shamor").
Oznayim la'Torah: Because whereas now, after hearing the first two Dibros from Hashem, they reached a high level of Ru'ach ha'Kodesh and were able to see what one normally hears 1 - they were able to see the Creation as if they had actually been there, in Va'eschanan, which is speaking after the sin of the Golden Calf, Shabbos could only remind them of Yetzi'as Mitzrayim, which they witnessed firsthand.
As the Torah writes in Pasuk 16 "ve'Chol ha'Am Ro'im es ha'Kolos".
