Question: Is there any basis for the custom of covering knives at the end of a meal before Birkat Hamazon? Similarly, is there any difference between Shabbat and weekdays regarding this law?
Answer: The custom to cover knives on the table with a napkin and the like at the end of a meal before Birkat Hamazon is based on the words of Maran Ha’Bet Yosef (Chapter 180) who quotes the Sefer Ha’Roke’ach, as follows: “Knives are covered during Birkat Hamazon because the verse states (regarding the Altar), ‘You shall not wave metal over them.’ The Mechilta that the reason why metal may not be waved over the Altar is because the Altar serves to lengthen one’s life and it is improper that a knife which shortens one’s life be waved over an object that lengthens one’s life. One’s table symbolizes the Altar, as stated in Masechet Chagiga (27a).” Rabbeinu David Abudirhem and the Shiboleh Ha’Leket write accordingly.
The Shiboleh Ha’Leket adds: “I have heard another explanation from the scholar, Rabbi Simcha, regarding this law which is that once, when a certain individual was reciting Birkat Hamazon and reached the ‘Boneh Yerushalayim’ blessing, he was reminded about the destruction of the bet Hamikdash. Out of agony, he took the knife (from the table) and stabbed himself in the stomach with it. It has therefore been customary since then to remove knives from the table before Birkat Hamazon.” Both of these reasons are likewise recorded in the Orchot Chaim. Maran Ha’Shulchan Aruch concludes that nevertheless, it is customary not to remove knives from the table on Shabbat and holidays; although according to the reason offered by Rabbeinu Simcha there is no reason to differentiate between Shabbat and weekdays, the custom of the Jewish nation becomes law. (The Magen Avraham writes that according to the first reason we have quoted, it is understandable why knives are not removed on Shabbat, for the Altar may not be built on Shabbat and there is thus no correlation to the building of the Altar upon which a knife may not be waved. The first reason is the more halachically correct one.) There is a deep Kabbalistic reason for this as well.
Maran rules likewise in his Shulchan Aruch that it is customary to cover knives before Birkat Hamazon; however, this is not the custom on Shabbat. Rabbeinu Mordechai Yaffeh, author of the Sefer Ha’Levush, writes that their custom is not to differentiate between Shabbat and weekdays and the knife is always covered, for they are concerned with the reason of Rabbeinu Simcha according to whom Maran Ha’Bet Yosef writes there is no distinction between Shabbat and weekdays. Nevertheless, the Turei Zahav writes that even according to the reason of Rabbeinu Simcha, there is room to differentiate between Shabbat and weekdays, for whereas during weekdays one is truly pained over the destruction of the Bet Hamikdash (especially in earlier generations when people were tremendously pained by the destruction of the Temple and the exile of the Jewish nation), on Shabbat one is not as pained and this is not such a concern.
Maran Ha’Chida writes in the name of the great Mekubalim the saintly Rabbeinu Moshe Cordovero and his (the Chida’s) grandfather, Hagaon Harav Avraham Azulai, that on the contrary, knives should not be covered on Shabbat and Yom Tov, for according to the Kabbalah, Shabbat symbolizes the World to Come when the harsh judgment on Earth will be sweetened and there will no longer be any death leaving no reason to cover knives on Shabbat.
Summary: It is customary to cover knives at the end of a meal before reciting Birkat Hamazon. Those who customarily do not cover knives on the table on Shabbat or holidays or those who do not have a clear custom regarding this issue should preferably not cover knives on Shabbat and holidays.
