Positive commandment · Mitzvat Aseh #59 of 613
Blowing the trumpets in the Temple and in war
לתקוע בחצוצרות על הקרבנות ובשעת הצרות
The verse · Bamidbar 10:10
וּבְיוֹם שִׂמְחַתְכֶם וּבְמוֹעֲדֵיכֶם וּבְרָאשֵׁי חׇדְשֵׁיכֶם וּתְקַעְתֶּם בַּחֲצֹצְרֹת עַל עֹלֹתֵיכֶם וְעַל זִבְחֵי שַׁלְמֵיכֶם וְהָיוּ לָכֶם לְזִכָּרוֹן לִפְנֵי אֱלֹהֵיכֶם אֲנִי ה׳ אֱלֹהֵיכֶם
“Also in the day of your gladness, and in your solemn days, and in the beginnings of your months, you shall blow with the trumpets over your burnt offerings, and over the sacrifices of your peace offerings; that they may be to you for a memorial before your God: I am the Lord your God.”
The Rambam’s words
To sound the trumpets when the sacrifices [are offered] and in times of difficulty, as [Numbers 10:10] states: "And you shall sound the trumpets...."
לתקוע בחצוצרות על הקרבנות ובשעת הצרות, שנאמר: "וּתְקַעְתֶּם בַּחֲצוֹצְרֹת" (במדבר י, י).
Why · Sefer HaChinuch #384
Blowing the trumpets in the Temple and in war
“It is from the roots of the commandment that [it is] since at the time of the sacrifice they would need to properly focus their attention — as it is well-known that it is disqualified by certain thoughts — and also [that] the sacrifice requires complete awareness in front of the Master of all Who commanded about them; and also at the time of trouble, a person needs great focus in his supplication before his Creator, that He should have mercy upon him and save him from his trouble; we are therefore commanded to blow the trumpets at these times.”
Who & when
“The commandment is practiced by priests at the time of the Temple, as the commandment to blow the trumpets is incumbent upon them, and like the matter that is written about them in the movements of the encampments (Numbers 10:8), “And the sons of Aharon, the priests, will blow with the trumpets.””
Where in the Mishneh Torah
Hilchot Fasts הלכות תעניות