Why does the Torah insert Shabbos in the Parshah of Mo'adim?
Rashi: To teach us that, on the one hand, if someone desecrates Yom-Tov, it is as if he desecrated Shabbos, and on the other, if he observes Yom-Tov, it is as if he observed Shabbos. 1
Ramban #1: Because Shabbos also falls under the category of 'Mo'ed' and can therefore be termed "Mikra Kodesh." 2
Ramban #2: "Mo'adei Hashem" (in Pasuk 2) refers specifically to the Yamim-Tovim listed later. 3 And the Torah is coming to draw a distinction between the Mo'adim of Beis-Din and Shabbos, inasmuch as, whereas only Meleches Avodah is forbidden on the former, all Melachah is forbidden on the latter. 4
Ramban #3: It is coming to teach us that if Yom-Tov falls on Shabbos, it does not override it to permit cooking - rather, Shabbos remains intact. 5
Seforno: The Torah is simply drawing a distinction between Shabbos, which has been fixed since the Creation, 6 and Yom-Tov, which is fixed by Beis-Din. 7
Divrei Eliyahu: Here, 'Shabbos' refers to Yom Kipur, when all Melachah is forbidden. The 'six days' refers to the other Yamim Tovim, on which Melachah of Ochel Nefesh is permitted.
See Torah Temimah, note 8.
Ramban: The Torah goes on to separate the Yamim-Tovim from Shabbos, by inserting the words "Asher Tikre'u Osam b'Moadam" (23:4), which it did not do in Pasuk 2.
Ramban: And it is because the Torah interrupted with Shabbos that in Pasuk 4, it repeats "Eileh Mo'adei Hashem".
Ramban: A clear proof that Shabbos is not included in the Mo'adim lies in the fact that the Pasuk does not mention Korban by Shabbos, and then at the end of the Parshah (in Pesukim 37 & 38) it concludes "Eileh Mo'adei Hashem ... Mikra'ei Kodesh Lehakriv Isheh La'Shem ... Mil'vad Shabesos Hashem". See Ramban, who brings a second proof.
Ramban: Similar to Shemos 35:1, 2 (See Rashi there).
Seforno: Pesachim 117b.
Seforno: As the Gemara states in Rosh Hashanah 25a - 'You (Beis-Din, your declaration of Rosh Chodesh is valid) even be'Shogeg! You, even be'Mezid! You, even if you were tricked!'
Why does the Torah insert the (otherwise superfluous) word, "Shabbos Hi"?
Yerushalmi Shabbos, 7:1: To render someone who transgresses a number of Shabbasos, Chayav (a Chatas) for each Shabbos independently. 1
See Torah Temimah, note 9.
What are the implications of "Shabbos Hi La'Shem"?
Yerushalmi Shabbos, 15:3: It implies that one should learn Torah on Shabbos. Consequently, coupled with the Pasuk in Pinchas (Bamidbar 29:35) - in connection with Shemini Atzeres 1 - "Atzeres Tih'yeh Lachem," which implies that one should eat and drink on Shabbos, one should spend part of Shabbos studying Torah and part eating and drinking. 2
"Shabbos Hi La'Shem". How will we reconcile this with the Pasuk in Bamidbar 29:35 - in connection with Shemini Atzeres* - "Atzeres Tih'yeh Lachem" - implying that it is a Mitzvah to eat and drink on Shabbos?
Yerushalmi Shabbos, 15:3: This teaches us that one should spend part of Shabbos studying Torah and part, eating and drinking. 1
Even though the Pasuk in Bamidbar is discussing Shemini Atzeres. See Torah Temimah, note 10.
Why does the Torah insert the words "Shabbas Shabbason"?
Ramban: Since, as opposed to Yom-Tov, where the Torah in Bo (Shemos 12:16) permitted Meleches Ochel Nefesh, the Torah here writes "Kol Melachah Lo Sa'asu," implying even what is necessary for Ochel Nefesh. Consequently, it refers to Shabbos as "Shabbas Shabbason" - even from Meleches Ochel Nefesh. 1
Ramban: And it is for the same reason that in Pasuk 32, the Torah refers to Yom Kipur too as "Shabbas Shabbason." See Oznayim la'Torah who explains how, for the same reason, the Torah refers to Shemitah as "Shabbas Shabbason" (25:24).
Why does the Torah insert the words, "be'Chol Moshevoseichem"?
Ramban #1 (on Pasuk 15, citing the Mechilta) and R. Bachye: To exclude Hashem's dwelling (the Beis-ha'Mikdash) 1 from the prohibition against making a fire. 2
Ramban #2 (on Pasuk 15, citing Kidushin 37b): To teach us that, even though the Torah inserts Shabbos in the Parshah of Mo'ados, it does not require the Beis-Din to sanctify it. 3
Seforno: Even though the length of a day differs from place to place, 4 we must keep Shabbos from nightfall to nightfall according to the way Hashem who created light and darkness fixes it in that location.
Ramban: Where it is permitted as an integral part of the Avodah.
Which explains why the Torah does not insert 'Chukas Olam l'Doroseichem' - See R. Bachye.
Ramban: Because it was fixed from the time of the Creation ('Shabbos Kevi'a ve'Kayma'). See also Torah Temimah, notes 11 & 12, and Oznayim la'Torah who concludes that this Derashah is an Asmachta.
Seforno: And despite the fact that it sometimes differs from the length of the first Shabbos of the Creation.
Why does the Torah write "Tei'aseh Melachah" (passive) and not 'Ta'aseh Melachah'?
Moshav Zekenim (Shemos 35:2): When Yisrael merits, their work is done through others.
Oznayim la'Torah #1: To teach us that, if one desists from working on Shabbos, one's work during the week will pass smoothly - as if it was being done by itself.
Oznayim la'Torah #2: It is warning that we are obligated to see to it that work is confined to six days a week, but is not performed on Shabbos.
