On Shabbat evening, when we come home, we sing a song that begins: “Peace be upon you, ministering angels, angels on high…”, “Come in peace…”, “Bless me with peace…”, and ends: “Depart in peace, angels of peace…”.
This is puzzling: why do we tell the angels to leave? Why shouldn't they stay? How are they in the way?
The Chofetz Chaim asks this question and answers that we welcome the Shabbat angels and bid farewell to the angels of the weekdays. This is like what our forefather Jacob saw in his prophetic dream: the angels of other nations left, and the angels of the people of Israel arrived.
However, this question can also be answered differently. When a kohen [even the High Priest] performed the regular service in the Beit HaMikdash, other people were not forbidden to be in the same place at that time. However, when the Kohen Gadol—the High Priest—enters the Holy of Holies to perform the service, the Torah commands: “No one shall be in the Tent of Meeting when he enters to make atonement in the Sanctuary, until he comes out” (Vayikra, 16:17), and the Sages add in the Midrash: “No one, not even those of whom it is written: ‘their faces are like a human face’”. In other words, even the angels were forbidden to be there during the High Priest's service. Why?
Because this is a moment of connection between the Almighty and the people of Israel, a moment of closeness that requires absolute privacy. So even the angels, with all due respect for their greatness and holiness, must leave this place!
When we leave the synagogue on Shabbat evening, the angels accompany us home. We arrive and find the table set and ready for the meal. We stand up and say: “Peace be upon you, ministering angels… come in peace…”, and also: “Bless me with peace…”.
Then it is as though we say to them: dear angels, we are now about to sanctify Shabbat with Kiddush over wine and begin the Shabbat meal, which possesses the highest level of holiness. At this meal, “no one shall be in the Tent of Meeting,” and so—“depart in peace, angels of peace”.