The Rule of St. Benedict - Chapter 10. The Arrangement Of The Night Office In Summer

(1) From Easter until the first of November, the winter arrangement for the number of psalms is followed.
(2) But because summer nights are shorter, the readings from the book are omitted. In place of the three readings, one from the Old Testament is substituted. This is to be recited by heart, followed by a short responsory. In everything else, the winter arrangement for Vigils is kept. Thus, winter and summer, there are never fewer than twelve psalms at Vigils, not counting Psalms 3 and 94.

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