Why does it say "v'Gam"?
Radak: This is a nice gift that Hashem gave to His nation Yisrael to remind them of the day of rest that He gave to them, and that He took them from slavery to freedom.
Why does it say "Shabsosai" (plural)?
Radak: This includes resting on Mo'adim; also they commemorate Yetzi'as Egypt
How is Shabbos an Os?
Rashi: It is a great sign that I gave to them My day of rest for rest. This shows that I sanctified them to Me!
Radak: It shows that He sanctified them and separated them from the nations. He separated them from the idols of Egypt, and gave to them Kadosh Torah, Chukim and proper Mishpatim.
Malbim (12-13): It is testimony of Emunah - Chidush (new matters) and Hashgachah Pratis that clings to Yisrael, to do for them wonders and override nature. It is a sign between Me and them of Hashgachah and Elokus dwelling among them. They must know that from My side, I, Hashem, am Mekadesh them, and they must be Mekadesh themselves from physicality to spirituality and Kedushah, and cling to Divinity.
