Some people have the custom to cover up knives during Birkat Hamazon, Grace after Meals. Commentators explain that just as it is forbidden to use metal tools to build an altar (Exodus 20:22), it is forbidden to have metal tools on the table while reciting Birkat Hamazon.
The Talmud compares a table to an altar (Chagiga 27a). An altar is there to lengthen a person's life and metal, ie weapons, shortens one's life (Mechilta on Exodus 20:22). So too, Grace after Meals lengthens our life. Therefore, some cover or even remove knives from the table.
The Shulchan Aruch rules that this custom does not hold on Shabbat or festivals, although some commentators dispute that.
Rav Ovadia Yosef explained that Shabbat is a taste of the world to come, where there will be no death. On Shabbat, knives do not express the potential for harm or violence.