"You must use the psalms of the day at Sext" - why? I've recently taken to praying the Liturgy of the Hours using the Universalis program. This is fabulous in all kinds of ways. Today, however, I found something that I didn't understand.

Normally at the daytime offices (Terce, Sext, None) you can choose whether to say the psalms of the day or the complementary psalms (which are always the same). Today, however, there is an instruction at the psalms for Terce and None:


  SPECIAL FOR TODAY: If you are celebrating Sext today, you must use the psalms of the day at Sext, and use the complementary psalms now.


There is a similar instruction on Wednesday, but this time the psalms of the day are to be used at None  instead.

Why are there these specific instructions about which sets of psalms to use on these certain occasions?
I alerted Martin Kochanski, the author of Universalis, to this question, and he's written a blog post specially for it, to which readers are referred.

The simple answer is: on certain days in the four-week cycle, the Breviary has a small and slightly obscure rubric whose effect is that the complementary psalms aren't allowed at a certain daytime Hour.
No reason is ever given, but when I was programming Universalis I finally managed to work it out. On those days, some of the complementary psalms in question appear in one of the main Hours. For instance, on Monday of the third week of the four-week cycle, Psalm 70(71) is recited at Vespers. Psalm 70(71) is also one of the complementary psalms for Sext — so, to avoid repetition, the Breviary says &quot;don't use the complementary psalms at Sext&quot;.

Similar things can happen at Lauds with the Invitatory psalm, and even in the Anglican Book of Common Prayer, where some psalms which are alternatives to canticles are prohibited where they occur in the ordinary course of reciting the psalter.