(Sorry, from last week's parashah)
Lev. 25:9: "Then you shall have the shofar sounded loud; on the tenth day of the seventh month—on the Day of Atonement—you shall have the shofar sounded throughout all your land."
Rashi ad loc. asks:
On the Day of Atonement -- since it is said, "on the Day of Atonement," do I not know that it is on the tenth of the month? If so, why was it said,The Torah specifies that the shofar on Yom Kippur and on the tenth day of the seventh month to make it clear that we must blow the shofar even when that day falls out on Shabbos.
"On the tenth day of the seventh month"? Rather, to tell you [that] blowing [the shofar] on the tenth of the month supersedes the Shabbos in all your land...
However, one can ask that playing a musical instrument on Shabbos is biblically permissible. It is only prohibited on Shabbos and Yom Tov rabbinically, due to the concern that one might fix the instrument. And even blowing a shofar on Rosh Hashanah that falls out on Shabbos is only rabbinically prohibited because of "Gezeirah De-Rabbah," the concern that one might bring it to an expert.
If that is the case, and one is biblically permitted to blow a shofar on Shabbos, why would the Torah have to instruct us to specifically blow it on the Yom Kippur of the Jubilee even if it falls on Shabbos? Biblically, one can blow it on any Shabbos!
The Ramban (ad loc.) asks this question, and the Gur Aryeh answers that this entire discussion is only an asmakhta and not meant to be the literal intent of the Torah.
I would suggest, however, that perhaps the following could explain the Rashi. The Taz asked why we perform circumcision on Shabbos and are not concerned with "Gezeirah De-Rabbah." He famously answered that because the Torah specifically tells us to perform a circumcision even on Shabbos, the Sages have no authority to forbid it. The Sages are unable to contradict an explicit biblical permission (see this post and this post).
Based on this, one could suggest that the Torah was explicitly permitting blowing the shofar on the Yom Kippur of the Jubilee that falls on Shabbos to prevent the Sages from ever prohibiting it.