Jamieson, Fausset and Brown on Joshua 6:3-5

3-5. ye shall compass the city, all ye men of war. ... Thus shalt thou do six days, &c.--Directions are here given as to the mode of procedure. Hebrew, "horns of jubilee"; that is, the bent or crooked trumpets with which the jubilee was proclaimed. It is probable that the horns of this animal were used at first; and that afterwards, when metallic trumpets were introduced, the primitive name, as well as form of them, was traditionally continued. The design of this whole proceeding was obviously to impress the Canaanites with a sense of the divine omnipotence, to teach the Israelites a memorable lesson of faith and confidence in God's promises, and to inspire sentiments of respect and reverence for the ark as the symbol of His presence. The length of time during which those circuits were made tended the more intensely to arrest the attention, and to deepen the impressions, both of the Israelites and the enemy. The number seven was among the Israelites the symbolic seal of the covenant between God and their nation [Keil, Hengstenberg].