According to Jewish tradition, shofar blowing is suspended when Rosh Hashanah falls on **Shabbat**.
The shofar is central to Rosh Hashanah worship: its blasts recall biblical trumpet signals, proclaim divine kingship, and traditionally awaken people to repentance during the High Holy Days. Yet the usual public shofar ritual is not performed on Shabbat, even when the first day of Rosh Hashanah coincides with the Sabbath.
This practice is connected to rabbinic concern that someone might carry a shofar to an expert for instruction, an activity traditionally prohibited on Shabbat. The Mishnah records that shofar blowing was once restricted on Shabbat outside the Temple, while Rabban Yohanan ben Zakkai later permitted it at Yavneh and nearby places. Rabbinic Judaism ultimately retained the general suspension.
The rule does not mean that the shofar is unimportant that year. When Rosh Hashanah lasts two days, communities ordinarily blow the shofar on the second day if it is not Shabbat. This is also why “Shabbat” is more precise than choices such as Monday: the suspension is tied to the Sabbath, not to a particular weekday name.