What is the meaning of "Veha'avartem Shofar Teru'ah"?
What is the reason for blowing the Shofar?
Seforno: It is an expression of joy at the releasing of all the Avadim Ivrim, and at the return of all fields (and houses not in walled cities) to their original owners.
Having stated "be'Yom ha'Kipurim", why does the Torah see fit to add "be'Asor la'Chodesh"?
Rashi: To teach that blowing the Shofar on the tenth of the month overrides Shabbos throughout the land, but not blowing the Shofar on the first (on Rosh Hashanah), which overrides Shabbos only in Beis-Din. 1
Ramban: Rashi is teaching us that blowing the Shofar on Yom Kipur of Yovel is incumbent, not only on Beis-Din (like counting to Yovel), but also upon every individual. His explanation is Halachically correct, but he did not differentiate between what is Min ha'Torah and what is merely an Asmachta mid'Rabanan.
QUESTIONS ON RASHI
Rashi writes that "be'Asor la'Chodesh" teaches that blowing the Shofar on the 10th overrides Shabbos throughout the land, but on Rosh Hashanah, only in Beis-Din. Blowing the Shofar is a Chachmah, and not a Melachah (Rosh Hashanah 29b)!
Moshav Zekenim: This is a mere Asmachta. 1 It does not override Shabbos due to a decree lest one carry a Shofar four Amos in Reshus ha'Rabim.
Moshav Zekenim: In the Havah Amina that it is mid'Oraisa, the Gemara expounded "Zichron Teru'ah" (sometimes we do not blow, i.e. when Rosh Hashanah is on Shabbos). It did not learn from "be'Asor la'Chodesh"!