Fresh Bread: The Original Baker’s Dozen

One of my daughters loves the Shabbos zemer “Ki eshmerah Shabbos keil yishmereini, os hi l’olmei ad beino u’veini…” It was written by Rav Avrohom Ibn Ezra. For some reason, some bentchers leave out this particular zemer. We have a fancy set of bentchers that does not have this song. I find myself augmenting our table with alternative bentchers. In any case, there is a line that I must have recited many times without understanding the proper meaning:
“Rasham b’das ha’keil chok el s’ganav, bo la’aroch lechem panim b’fanav, al kein l’hisanos bo al pi nevonav asur, l’vad miYom Kippur avoni—He inscribed in the G-dly law a decree for his priests that on it [the day of Shabbos] they prepare showbread before him. Therefore it is forbidden to fast on it by order of his understanding sages except for the day when my sins are atoned” (translation courtesy of ArtScroll).
The twelve loaves of lechem ha’panim stayed on the Shulchan from Shabbos to Shabbos. On Shabbos the old batch would be removed and a new batch put on. The kohanim were then commanded to eat the old batch. Hence, they couldn’t fast on Shabbos because then they would be derelict in their fulfillment of the mitzvah of eating the lechem ha’panim. The exception was when Yom Kippur fell out on Shabbos. That Shabbos they were not allowed to eat the showbread.