What is "Ki Sheishes Yamim ... " referring to?
Seforno: The objective of Shabbos is to emulate Hashem and to be like Him
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.
What does "Vayanach" mean vis-a-vis Hashem?
Rashi: Hashem used the term 'rest' regarding Himself for us to learn a Kal va'Chomer - Man, who toils and becomes weary, should certainly 'rest' (cease working) on Shabbos.
Rosh: It means 'He ceased'. 1
What blessing did Hashem confer upon the Shabbos and how did He sanctify it?
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
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).
Here, the Torah says that we must observe Shabbos because Hashem created the world in six days. In Va'escchanan, Devarim 5:15, it says due to Yetzi'as Mitzrayim?
Moshav Zekenim: Really, it is due to creation. Lest one say that he loses through resting on Shabbos, it says in Devarim that you were slaves, and never worked for yourselves. All the more so you should desist on Shabbos and Yom Tov for Me.
Commentaies: 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
As the Torah writes in Pasuk 16 "ve'Chol ha'Am Ro'im es ha'Kolos".