According to the relevant section of the 1928 Prayer Book, Proper Psalms are only mandatory on 13 particular holy days, plus their eves, where applicable. It also says that Psalms and lessons for morning and evening are interchangeable; that on Sundays, the Epistle or Gospel may be read instead of the Second Lesson; lessons may be lengthened or shortened at the minister's discretion; and that on "special occasions," the minister may select Psalms and lessons at will. The fact that it specifically says that the officiant may choose Psalms at will on nearly all occasions, but doesn't say the same for Lessons (except on "special occasions") suggests that regular Sunday Mattins and Evensong must have some set of the approved readings. For example, yesterday (Trinity XIV) would have one of the six pairings listed in the Lectionary: Micah 6/Philippians 4 1 Chronicles 29/Luke 17 Genesis 45/Romans 12 Jeremiah 7/Luke 13 Deuteronomy 8/Luke 17 1 Kings 3/Acts 17 ...And any of the New Testament lessons can be replaced with Ephesians 4 or Luke 14. This is definitely a lot of variety, and because of that, the argument can be made both ways: The Prayer Book already gives you 18 options. Why do you need more than that? Do your parishioners really need to hear that sermon series on Song of Songs you've been thinking about? The Prayer Book already gives you 18 options. It's definitely in the spirit of those guidelines for you as a rector to choose such lessons you think are appropriate and edifying for your parish, whether they're ordered in the Lectionary or not. Which perspective was more in line with actual practice, with regard to Mattins and Evensong?