Numbers 10:8-10

8The sons of Aaron, the priests, are to sound the trumpets. This shall be a permanent statute for you and the generations to come.

9When you enter into battle in your land against an adversary who attacks you, sound short blasts on the trumpets, and you will be remembered before the LORD your God and saved from your enemies. 10And on your joyous occasions, your appointed feasts, and the beginning of each month, you are to blow the trumpets over your burnt offerings and peace offerings to serve as a reminder for you before your God. I am the LORD your God.”