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 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 extrapolate that blowing the Shofar on the tenth of the month overrides Shabbos throughout the land, 1 but not blowing the Shofar on the first of the month (on Rosh Hashanah), which only overrides Shabbos in Beis-Din. 2
Rashi is teaching us that blowing the Shofar on Yom Kipur of Yovel is incumbent, not only on Beis-Dm, but also upon every individual.
See Ramban, who points out that, although Rashi's explanation is Halachically correct, he fails to differentiate between what is min ha'Torah and what is merely an Asmachta mi'de'Rabbanan.