For a little over 200 years, Psalm 27 has been associated with the season of repentance: Some have the custom of reciting this psalm during Days of Awe (10 days), some for the whole month of Elul as well (40 days), and some beginning on Rosh Hodesh Elul and continuing through Hoshana Rabba (51 days). There are several explanations for this association. Most focus on the psalm’s themes; also noted: the expression “were it not” — לוּלֵא — in verse 13 spells Elul — אלול — backward.
Many siddurim include the full psalm somewhere in Psukei D’zimrah (verses of song, in the morning service). Mishkan T’filah includes the single verse, 27:4, for which there are a number of popular tunes (p.662 in “songs and hymns”).