What is the connection between Shemitah and Har Sinai - bearing in mind that all the Mitzvos were said at Sinai?
Rashi (citing Toras Kohanim): To teach us that, just as Shemitah was given at Sinai Klalosehah, Pratosehah and Dikdukehah (its general rules, details and fine points), so too, were all the Mitzvos given at Sinai with their Klalim and Dikdukim. 1
Rashbam: It teaches us that Shemitah was said at Har Sinai, before the Ohel Mo'ed was erected.
Hadar Zekenim, Da'as Zekenim, Rosh (20): All of Sefer Vayikra until now was said in the Ohel Mo'ed and pertains to the Ohel, i.e. Korbanos, Metzora, Temei'im, Kehunah
Why does the Torah insert the Mitzvah of Shemitah here?
Ramban #1 (citing Ibn Ezra): This is part of the Bris with Yisrael (Shemos 24:7). 1 It is taught here to connect it with Aveiros for which they can be exiled from the land - the Arayos, in Acharei-Mos and Kedoshim, and letting the land rest in Shemitah. 2
Ramban #2: Har Sinai in this Pasuk is referring to the second time that Moshe ascended the Mountain to receive the second Luchos, and that is when Hashem taught him about the Shemitah. 3
Moshav Zekenim citing R. Yehudah ha'Chasid: It is taught next to the blasphemer, because (a) modest Chachamim teach their Talmidim Hashem's name once in Shemitah (seven years), (b) the blasphemer's Pasul lineage was taught; one may not deposit Peros with someone suspected about Shemitah; 6 (c) if you will ask, from where did the blasphemer know Hashem's name
Ramban: When they declared "Na'aseh v'Nishma".
Ramban: See 18:28, 26:34. He inserted Emor first, in order to mention the Shabbasos before the Shemitah. .
Ramban: Immediately following the erection of the Mishkan, Hashem called Moshe (Vayikra, 1:1) and commanded him the Parshah of Korbanos and all of Toras Kohanim, which Moshe immediately taught to Yisrael. He also told them that Hashem had commanded him on Har Sinai to explain to them the Shemitah and Yovel, and to enter into a (new) covenant with them on all the Mitzvos and Mishpatim with a curse and an oath. This covenant did not require the Korbanos that the first one did; they did however, reaccept the original covenant with the oaths and the curses (mentioned in Bechukosai - see 26:46), because Hashem pardoned them (for the sin of the Eigel) on these conditions, as I explained (Shemos 34:47). In fact, Yisrael reaffirmed the B'ris again under the same conditions at Arvos Mo'av (See Devarim 28:69). Moshav Zekenim
Rashi brings from Toras Kohanim that just like Shemitah was given at Sinai
Rashi: They were not repeated at Arvos Mo'av (in Sefer Devarim), so we must say that they were totally taught at Sinai. 1
Ramban: Klalosehah (its general rules) refers to "veha'Shevi'is Tishmetenah u'Netashtah" (Shemos 23:11). Pratosehah and Dikdukehah (its details and fine points) are taught here. And "Eileh ha'Mitzvos... " (27:34) equates all the Mitzvos to Shemitah. 2
Ramban: Many Mitzvos were not repeated in Arvos Mo'av! Perhaps those that were repeated, this was to add matters not taught at Sinai! Moshav Zekenim