As is known the custom is to say this chapter of Tehilim (27) throughout the month of Elul until after Yom Kippur, yet a reason is not provided.
Perhaps, the custom is based on Medrash Rabbah in Parshas Acharei, that אורי refers to Rosh HaShana, and ישעי refers to Yom Kippur. And the extension is to say this chapter of Tehilim also before and after those 10 days which became the general custom.
One might understand better by reference to a Medrash in Parshas BeShalach that this Chapter of Tehilim alludes to the Splitting of the Sea according to Rabbi Eliezer.
- ים סוף (שמות יד וט"ו ) (תהלים כז) לדוד ה' אורי
- התיצבו וראו את ישועת ה' (יד, יג) אורי וישעי (תהלים כז, א )
- ויאמר משה אל תיראו (יד,יג) ממי אירא )א(
- עזי וזמרת יה (טו, ב) ה' מעוז חיי )א(
- תפול עליהם אימתה ופחד (טו, טז) ממי אפחד )א(
- ופרעה הקריב (יד, י) בקרוב עלי מרעים )ב(
- אמר אויב ארדוף אשיג וגו' (טו, ט) לאכול את בשרי )ב(
- ונער פרעה וחילו צרי ואויבי לי (ב)
- מחנה של מצרים אם תחנה עלי מחנה )ג(
- מלחמתן של מצרים לא יירא לבי אם תקום עלי )ג(
- בזאת שהבטחתני שנאמר "ה' ילחם לכם" (יד, יד) בזאת אני בוטח )ג(
Rabbi Epstein is surprised that we do not say this Tehilim on Pesach, or at least on the seventh day of Pesach, since this entire Tehilim alludes to the incident that occurred on that day.
ועקדת יצחק לזרעו היום ברחמים תזכור
The Shulchan Aruch (O.C. 591:7) says this is the correct nusach and not the nusach that adds the name of Yaacov (in order not to include Yitzchak’s children which includes Eisav).
The nusach can be understood according to the Medrash about Tehilim (22, 24) הללוהו כל זרע יעקב כבדוהו, וגורו ממנו כל זרע ישראל. The Medrash explains that the first part of the verse refers to the 11 sons of Yaacov, and the second part of the verse refers to Binyomin son of Yisrael. Eleven sons were born when he was known as Yaacov, and only Binyomin was born when he was called Yisrael.
Thus, the above phase cannot refer to Yaacov nor refer to Yisrael since either would not include all of the Jewish tribes. Thus, the verse leaves the word without specific reference, just זרעו.
As a side note, one can understand the verses in Parshas BeShalach:
- (שמות יד,טו) דבר אל בני ישראל ויסעו
- (טז) ויבאו בני ישראל בתוך בים ויבאו בני ישראל.
The second B’nai Yisrael seems extra. The second half of the verse also refers to the entering into the sea. Thus, the second B’nai Yisrael refers to the tribe of Binyomin (and not Yehudah) that had the merit to be the first tribe to enter into the sea. This reflects that Binyomin was the honored one of the sons that he was born to Yisrael, who wrestled with angels and prevailed.
