The Rule of St. Benedict - Chapter 18. The Order Of The Psalmody

(7) On Monday at Terce, Sext and None, the remaining nine sections of Psalm 118 are said, three sections at each hour.
(8) Psalm 118 is thus completed in two days, Sunday and Monday.
(9) On Tuesday, three psalms are said at each of the hours of Tece, Sext and None. These are the nine psalms, 119 through 127.
(10) The same psalms are repeated a these hours daily up to Sunday. Likewise, the arrangement of hymns readings and versicles for these days remains the same.
(11) In this way, Psalm 118 will always begin on Sunday.

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