Why does the Torah mention specifically plowing and harvesting?
Rashi #1: With reference to Erev Shevi'is and Motza'ei Shevi'is, the Torah is teaching us the obligation of adding to the Sh'mitah year 1 , by not plowing one's field already from the end of the sixth year and not harvesting one's crops at the beginning of the eighth year.
Rashi #2: With reference to Shabbos, it comes to teach us that reaping, like plowing, 2 is only forbidden when it is R'shus (voluntary) - but reaping barley for the Omer, which is a Mitzvah, overrides Shabbos.
Ramban: Because the livelihood of man depends on them.
Seforno: Because, just as not working on Shabbos brings B'rachah to the whole week, not plowing and reaping in the Sh'mitah brings blessing to the entire seven-year cycle. 3
Rashbam: The Pasuk, which is referring to working on the land, 4 is simply coming to prohibit plowing and reaping on Shabbos - in spite of the necessity, how much more so other Melachos.
Rashi (in Rosh Hashanah 9a): Even though the beginning of the Pasuk discusses Shabbos, the end must discuss Sh'mitah, for all Melachos are forbidden on Shabbos, and not just plowing and reaping!
Which is always voluntary, since there is no such thing as CHarishah shel Mitzvah. See Sifsei Chachamim.
See Bereishis, 4:12 and Mishlei, 12:11 (Rashbam).
Why does the Torah place Shabbos in the middle of the Yamim Tovim?
Ramban: It places it next to Pesach and Bechoros, which, like it, commemorate the creation of the world. 1 Moreover, like them, it commemorates Yetzi'as Mitzrayim. 2
Seforno: Refer to 34:18:2:1.
Oznayim la'Torah #1: Because, similar to th Yamim-Tovim, 3 keeping Shabbos is an amazing Segulah that atones for Avodah Zarah. 4
Oznayim la'Torah #2: Because Shabbos was given to Yisrael after Marah, between Pesach and Shavu'os.
What are the implications of "u'va'Yom ha'Shevi'i Tishbos"?
Moshav Zekenim citing the Ramban: It is a Mitzvas Asei to refrain from Melachah on Shabbos.
Rashi (in Rosh Hashanah 9a) writes that the end of the Pasuk must be discussing Shemitah, since all Melachos are forbidden on Shabbos. Perhaps the Pasuk is coming to teach us that plowing and reaping are only an Aseh (just like R. Nasan says that lighting a fire is only a La'av)!
Riva (citing R"M of Kutzi): Since it is not written near the Chiyuv Misah for Melachah, we cannot say that it comes to remove plowing and reaping from the K'lal.
QUESTIONS ON RASHI
Rashi writes that the current Pasuk obligates adding to the Shemitah year - before and after. But Rashi (in Devarim 31:10) derives from "Sh'nas ha'Shemitah be'Chag ha'Sukos" that Motza'ei Sh'mitah is called Sh'mitah due to Tosefes Shemitah?
Riva: Rashi In Devarim, is following the opinion which establishes the current Pasuk byu Shabbos. Both Pesukim are rquired: If we had only the Pasuk there, we would say that the fruit of Sh'mitah that is picked on Motza'ei Shabbos have Kedushas Shevi'is, but we would not forbid working the land for that fruit before or after Shemitah. 1
The Riva also asked why we need to derive Tosefes Yom Kipur from "ve'Inisem... be'Sish'ah" (Rosh Hashanah 9a), and he left the question unanswered (PF)
