(Halacha For Today – Ben Olam Haba)
Answer:
In the Sefer Boruch She’Amar (written by the author of Chumash Torah Temimah) he maintains that this minhag developed by mistake, based on a misunderstanding. The Posuk was written as “Migdol” and the letters “Shin” and “Bais” were written next to it as a source code meaning “Shmuel Bais”, However, over time people misinterpreted the “Shin Bais” as meaning “Shabbos” and thus began saying “Migdol” on Shabbos only and “Magdil” during the week.
The Ben Ish Chai ( Year 1, Parshas Chukas Siman 19) writes that this change to Migdol on Shabbos is “Al Pi Sod” , has a mystical reason.
The prevalent custom is to indeed say Magdil during the weekdays and Migdol on days that the Musaf prayers are said.
(Get daily halacha emails right to your email box by signing up at HalachaForToday@Gmail.com or HalachaForToday.com)
Ben says
For an extensive treatment of the Magdil/Migdol issue, see the article by Rabbi Dr Raymond Apple in the Jewish Bible Quarterly – http://www.oztorah.com/2013/03/magdil-migdol-liturgical-responses-to-textual-variants/