Halakhah zu Dewarim 27:5

It is customary to remove all the knives from the table before Birkas Hamazon or to cover them, because the table is like the Altar [of the Bais Hamikdash] and concerning the Altar it is said: "Do not lift up any iron upon them." ⁴Deuteronomy 27: 5. Since metal shortens human life [in the form of weapons] and the Altar prolongs human life, it is improper that something that shortens life be lifted above that which lengthens life. Similarly the table prolongs the days of man, and atones for his sins when needy guests are invited to the table, for the power of hospitality is so great that it causes the Divine Spirit to dwell in our midst. It is customary in many places not to cover the knives on Shabbos and Yom Tov; for on weekdays we cover them because they represent the power of Esau, but on Shabbos and Yom Tov the forces of evil do not prevail, and a custom of Israel has the validity of Torah.