107 Let us, then, investigate what we read in the Old Testament concerning the kinds of trumpets, considering that those festivals which were enjoined on the Jews by the Law are the shadow of joys above and of heavenly festivals. For here is the shadow, there the truth. Let us endeavour to attain to the truth by means of the shadow. Of which truth the figure is expressed in this manner, where we read that the Lord said to Moses: “Speak unto the children of Israel, saying, In the seventh month, on the first day of the month, shall be a rest unto you, a memorial of blowing of trumpets, it shall be called holy unto you.
You shall not do any servile work, and you shall kindle a whole burnt-offering unto the Lord.” And in the Book of Numbers: “The Lord spoke unto Moses, saying: Make you two trumpets of beaten work, of silver shall you make them, and they shall be to you for calling the assembly and for the journeying of the camp. And you shall blow with them, and all the congregation shall be gathered together at the door of the tabernacle of witness. But if you blow with one trumpet, all the princes and leaders of Israel shall come to you; and you shall blow a signal with the trumpet the first time, and they shall move the camp forward, and place it on the east.
And you shall blow a signal with the trumpet the second time, and they shall move the camp forward, and place it towards Libanus. And you shall blow a signal with the trumpet the third time, and they shall move the camp forward, which shall be placed towards the north [Boream]. And you shall blow a signal with the trumpet the fourth time, and they shall move the camp forward, which shall be placed towards the north [Aquilonem]. They shall blow a signal with the trumpet when they move forward.
And when you shall gather together the assembly, blow with the trumpet, but not the signal. And the sons of Aaron, the priests, shall blow with the trumpets, and it shall be for you a statute for ever throughout your generations. But if you shall go out to war into your own land, against the adversaries who resist you, you shall sound a signal with the trumpets and you shall be remembered before the Lord, and have deliverance from your dead. Also in the days of your gladness, and on your feast days, and on your new moons, you shall blow with the trumpets, and at your whole burnt sacrifices and at your peace-offerings, and it shall be for you for your memorial before the Lord, says the Lord.”
Source: On the Death of Satyrus (New Advent)