Numbers 10:8-10

8The sons of Aaron, the priests, must blow the trumpets; and they will be to you for an eternal ordinance throughout your generations.

9If you go to war in your land against an adversary who opposes you, then you must sound an alarm with the trumpets, and you will be remembered before the Lord your God, and you will be saved from your enemies.

10“Also in the time when you rejoice, such as on your appointed festivals or at the beginnings of your months, you must blow with your trumpets over your burnt offerings and over the sacrifices of your peace offerings, so that they may become a memorial for you before your God: I am the Lord your God.”