It’s an old custom to read (or to sing) the Book of Psalms during the month of Elul. In the famous Song of Moses, it is written: וַיּאמְרוּ לֵאמר אָשִׁירָה לַיהוָה / “and they spoke, saying: ‘I will sing to the LORD’ (Exod. 15:1). This phrase can be formed into an acronym for Elul (אלול), and the sages therefore reasoned that hearing the Psalms were vital during the Season of Repentance and Days of Favor.
Of all the great Psalms, however, Psalm 27 is considered the central one of the season of teshuvah. The midrash on the Psalms states that the word ori (אוֹרִי), “my light,” refers to Rosh Hashanah (based on Psalm 37:6) whereas the word yishi (יִשְׁעִי), “my salvation” (lit. “my Jesus”) refers to the atonement given on Yom Kippur. King David also mentions that God would hide him in his sukkah (בְּסֻכּה) in the time of trouble, referring to the holiday of Sukkot (Psalm 27:5). Therefore since it alludes to all three of the fall holidays, Psalm 27 is regarded as the thematic Psalm for the High Holidays of the Jewish year.
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