Reading or singing the Psalms has been a traditional element of Christian worship since the earliest days of the Christian church. In Presbyterian worship, it has been customary to use the psalms in two ways — to sing metrical paraphrases of them, such as those found at the beginning of the hymnbook (#1-108), and to read them responsively.
Like the other Scripture readings read in the service, the psalm for a given Sunday can be chosen in a number of ways. In our services, we generally read (or sometimes sing) the psalm specified for that day in the Revised Common Lectionary (a list of Scripture readings for every Sunday and holy day of the year). In the Revised Common Lectionary, the psalms are chosen generally to reflect the theme of the Old Testament reading for the day, and, for this reason, the psalm traditionally is read following the Old Testament reading, rather than preceding it as we do. Occasionally, the Lectionary specifies a poetic passage from outside the Book of Psalms, such as Isaiah 12 or Luke 1.46-55.