Les Nombres 10:10 Halakha : Mishneh Torah, Fasts, Mishneh Torah, Shofar, Sukkah and Lulav, Mishneh Torah, Positive Mitzvot & Sefer HaChinukh

It is a positive commandment from the Torah to cry out and to sound trumpets for all troubles that come upon the community; as it is stated (Numbers 10:9), "upon an enemy who attacks you and you sound trumpets." That is to say, [with] every matter that troubles you — such as famine, a plague, locusts and that which is similar to them — cry out about them, and sound the trumpets.

And likewise do we not decree a fast at the outset on [a] Rosh Chodesh or on Chanukkah or Purim or the intermediate days of a festival. But if they started to fast about a trouble even one day, and one of these [days] ran into [the order of the fasts], we fast and complete the day in fasting.

[When] the holiday of Rosh Hashanah falls out on Shabbat, we may not blow the shofar in any place. [This is the case] even though blowing [the shofar] was forbidden only on account of a Shabbat decree; so it would have been appropriate that we would blow — a positive commandment of the Torah would come and override a Shabbat decree from the words [of the Sages]. So why do we not blow? [Because of] a decree lest one take it in his hand and carry it to someone to blow for him, and [in the process,] carry it four ells in the public domain, or transfer it from one domain to another — and come to a prohibition [punishable by] stoning. [This is necessary because] all are obligated in blowing [the shofar], but not all are expert at blowing [it].