When Rosh Hashanah falls on Shabbath, people sometimes feel that something is lacking in view of the fact that we do not blow the Shofar on Shabbath. It says in Tokhahath Hayyim, that when Rosh Hashanah falls on Shabbath and there is no Shofar, we should rush to make Teshubah (repentance) before G-d, because this will awaken the trait of mercy in G-d, ten times more than the blowing of the Shofar.
Not only that, but we see from the fact that we do not blow the Shofar, how important it is to keep Shabbath in all its detail. Even though blowing the Shofar is a commandment from the Torah, and blowing it is not a Melakha (forbidden Shabbath labor), nevertheless, the Hakhamim forbade us to blow it on Shabbath out of the concern that someone may come to carry it in the public domain (which is a forbidden labor). If the Hakhamim prevented us from performing a positive commandment from the Torah out of fear that there might possibly be Hillul (desecration of ) Shabbath, we see how extremely particular we must be to not actually desecrate any Shabbath during the year.
When one keeps Shabbath in all its details, it is as if he has kept the entire Torah and it is worth more than 613 Shofars. So instead of feeling sad that there is no Shofar one should rejoice over the fact that we are keeping Shabbath correctly.
(See Tokhahath Hayim, Pereq 54)
