“The Rabbis forbade reading from Ketuvim1 on Shabbat in earlier times because of neglect of the study hall.” (Gemara Shabbat 115a)
Rashi explains that on Shabbat in olden times the Rav would deliver a public discourse that included aspects of practical halachah. In order to maximise attendance, the Rabbis disallowed the study of Ketuvim on Shabbat, since people might become engrossed in them and fail to attend the lecture.2 The Rabbis felt that as the lecture pertained to daily observances, it would be more beneficial for the public than Ketuvim. Therefore, they forbade the study of Ketuvim on Shabbat, although they later lifted this ban.
Based on this Gemara, the Rogatchover Gaon elucidates our enigmatic change in the text of Birkat HaMazon on Shabbat. During the week, we say “Magdil Yeshuot Malko”3 a verse from Psalms 18:51. On Shabbat though, we say “Migdol Yeshuot Malko”4 from Shmuel II, 22:51. Apparently, the difference originates from the period when it was forbidden to study Ketuvim on Shabbat. Since “Magdil” is from Ketuvim (Psalms), we replace it with “Migdol” (from a parallel verse in Neviim), since learning from Neviim on Shabbat is permissible4!
Rav Mordechai Kornfeld clarifies that even though there are many other verses from Ketuvim in the Shabbat liturgy, we may recite them because there is no other choice, as they do not have any close match in Neviim. Since they are part of our daily prayers, we are permitted to recite such quotes from Ketuvim. However, in Birkat HaMazon we change “Magdil” to “Migdol” in order to remind us of the prohibition against learning Ketuvim on Shabbat when not praying on Shabbat.
The commentator Etz Yosef provides an alternative explanation. Both verses, Magdil (He Who magnifies salvations for his king) and Migdol (He Who is a tower of salvations for his king), were written by King David and, in the context of Birkat HaMazon, king refers to King Messiah. The phrase from Psalms (Magdil) was chosen for weekdays because it was written before David became king. David composed the phrase from Shmuel (Migdol) when he was at the peak of his greatness, and it is therefore more suited to Shabbat and festivals.5
The Torah Temimah6 proposes a third hypothesis to justify the change in Birkat HaMazon. The change in the text may stem from a misreading of an abbreviation in the early printings of Birkat HaMazon. In the margin next to the word “Magdil” the following appeared in parentheses: “Migdol, Sh.B.” (the Hebrew letters “Shin” and “Bet”). The intention of that marginal note was that in Shmuel II (“Shmuel Bet” or Sh.B.), the word “Migdol” appears instead of “Magdil”. Later printers who copied from the earlier manuscripts misinterpreted the abbreviation to mean that “Migdol” is recited on Shabbat (which can also be abbreviated as Sh.B.).
1. Ketuvim comprises Psalms, Mishlei, Iyov, the five Megilot, Daniel, Ezra, Nechemiah and Divrei HaYamim.
2. Ketuvim are regarded as the most engaging part of Tenach, hence these books were read to the High Priest on the night of Yom Kippur to keep him from falling asleep. (Gemara Yoma 18b)
3. “He Who magnifies (or, is a tower of) salvations for his king, and does kindness for his anointed, to David and to his descendants for ever.”
4. The Rabbis did not ban the study of Neviim because the Haftarah was read from the Neviim.
5. Grammatically, Migdol relates to something that is finished and completed whilst Magdil relates to something that is ongoing. Hence, we say Migdol on Shabbat, because it is a day of rest and completion of the week, whereas we say Magdil during the week because our task of the week is still ongoing. (Rabbi Ginsbury)
6. Rabbi Baruch HaLevi Epstein (1860-1942)