Psalm 136 of the Psalter is one of the best known of the Biblical psalms. Its opening lines, "By the rivers of Babylon..." (Septuagint: "By the waters of Babylon...") have been set to music on several occasions.
The psalm is a hymn expressing the yearnings of the Jewish people in exile following the Babylonian conquest of Jerusalem in 586 BC. The rivers of Babylon are the Euphrates river, its tributaries, and the Chebar river (possibly the river Habor, the Chaboras, or modern Khabour, which joins the Euphrates at Circesium). In its whole form, the psalm reflects the yearning for Jerusalem as well as hatred for the Holy City's enemies with sometimes violent imagery. Rabbinical sources attributed the poem to the prophet Jeremiah, and the Septuagint version of the psalm bears the superscription: "For David. By Jeremias, in the Captivity."
The early lines of the poem are very well known, as they describe the sadness of the Israelites, asked to "sing the Lord's song in a foreign land". This they refuse to do, leaving their harps hanging on trees. The poem then turns into self-exhortation to remember Jerusalem. It ends with violent fantasies of revenge, telling a "Daughter of Babylon" of the delight of "he who seizes your infants and dashes them against the rocks." (New International Version).
Psalm 136 (which is known by its Hebrew numbering as Psalm 137) is a part of the Nineteenth Kathisma (division of the Psalter) and is read at Matins on Friday mornings throughout the year, except during Bright Week (the week following Easter Sunday) when no psalms at all are read. During most of Great Lent it is read at Matins on Thursday and at the Third Hour on Friday, but during the fifth week of Great Lent it is read at Vespers on Tuesday evening and at the Third Hour on Friday.
This psalm is also solemnly chanted at Matins after the Polyeleos on the three Sundays which precede the beginning of Great Lent.