It is customary to recite each day the "Shir Shel Yom," a special chapter of Tehillim designated for that particular day of the week. This Psalm is recited after Kaddish Titkabal toward the end of the Shaharit service every morning. Curiously, however, on Ereb Shabbat, we recite that day’s "Shir Shel Yom" (Tehillim 93 – "Hashem Malach Ge’ut Labesh") also during the Minha prayer, before Alenu.
What is the reason for reciting the "Shir Shel Yom" on Friday both during Shaharit and at Minha? Why is Friday different from the other days of the week in this regard?
The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in his work Od Yosef Hai (Vayakhel, 3), addresses this question and answers by explaining the nature and purpose of the daily "Shir Shel Yom" (listen to audio recording for precise citation). We recite this daily Psalm, he writes, in order to eliminate the prosecuting angels and harmful spiritual forces that threaten to sabotage our prayers, and to prevent them from ascending to the heavens. The "Shir Shel Yom" has the capacity to clear the air, so-to-speak, so that our Tefilot can ascend unharmed to the desired place in the heavens. Generally speaking, the Ben Ish Hai explains, the recitation of the "Shir Shel Yom" can accomplish this goal only during Shaharit, in the morning, which is governed by the divine attribute of Hesed (kindness). The prevalence of the attribute of Hesed empowers this recitation to break through the barriers that the forces of Din (judgment) set up to block our prayers. The afternoon, however, is the period of Din, when the attribute of strict justice prevails. During this period, the "Shir Shel Yom" is incapable of penetrating the harmful spiritual forces, which are stronger in these hours of the day. Therefore, we instead recite the Psalm of "La’menase’ah Bi’nginot Mizmor Shir (Tehillim 67), which is uniquely suited for the purpose of breaking through the barriers of judgment.
At Minha on Friday, however, when we have already begun entering the sanctity of Shabbat, the attribute of Hesed prevails. We are therefore able to recite the "Shir Shel Yom" to eliminate the harmful spiritual forces. This, the Ben Ish Hai explains, is the difference between Friday and other days of the week with regard to reciting "Shir Shel Yom" at Minha.
Incidentally, the Ben Ish Hai adds, this also explains why we do not recite the section of the Akeda during Minha. There were some Rabbis who advocated reciting the Akeda at Minha, noting that according to tradition, the event of Akedat Yishak actually occurred during the afternoon, and the afternoon is thus the more appropriate time to recall this event. The Ben Ish Hai explained that we do not recite the Akeda at Minha because, like the "Shir Shel Yom," these verses are recited for the purpose of eliminating the harmful forces that threaten to block our prayers. The event of Akedat Yishak, the binding of Yishak upon the altar, signifies the "binding" of the forces of judgment, as Yishak symbolizes the divine attribute of judgment. The verses of Akedat Yishak therefore cannot be recited in the afternoon, during the period of Din, when the attribute of strict justice prevails and empowers the harmful spiritual forces. During this period, we are best advised not to make any attempts to oppose the forces of Din.
Summary: We recite the "Shir Shel Yom" each morning because this recitation has the capacity to eliminate the harmful spiritual forces that threaten to sabotage our Tefilot. Generally, we recite this daily Psalm only in the morning, because the afternoon is the time of strict judgment, when the "Shir Shel Yom" does not have the strength to overpower the harmful spiritual forces. On Ereb Shabbat, however, as Shabbat approaches, the "Shir Shel Yom" is capable of breaking through these forces, and therefore we recite the "Shir Shel Yom" on Friday both at Shaharit and at Minha.
