I worked in a Roman Catholic Church for fifteen years and played for five Masses each weekend. While serving this parish, I really honed my knowledge and familiarity with the Lectionary. There are three years in a cycle. Year A, B and C. Each Sunday of each year has its own readings. For instance, a specific Sunday in year A will have three readings and a Psalm. Year B, the same calendar day will have different readings, and likewise for Year C. The collection of pre-selected readings come from a book called "The Lectionary." It is a collection of scripture organized and sorted for each Sunday of the year for three years. That means, every three years you will hear the same reading. There are however a few exceptions to the rule.
The priest at this church where I served for fifteen years took his homilies out of a book that some theologian wrote. No, his homilies were not his own, he did not write them, they were canned. But, they were good. I had my own personal copy of the Lectionary and during each Mass for the entire 15 years of service, I would scribble in the margins and stuff it with post-it notes about the music I used, what the congregation responded well to, what went well with the readings or the homily and what the homily was about, etcetera. Over the years Father would marvel about how flawlessly I could match the readings and even to his homily. He credited the Holy Spirit.
So in planning music, the first method I would consider is what I call "ACTS." - If I choose a hymn or song from each the the following categories, Adoration, Contrition, Thanksgiving and Supplication (ACTS), I can't miss. Many hymn books come with a thematic index. Even so, it isn't difficult to grasp the theme of a hymn by reading it carefully and prayerfully. Many hymns may also encompass multiple topics. When you choose hymns for the average liturgy in the order of ACST, you can't go wrong. That is one method.
I aslo take into consideration the season. If you schedule Christmas hymns during the Christmas season, most liturgist will be forgiving if it is casually chosen. The same applies to a Lenten song during Lent or an Advent song during Advent (which is not Christmas). Of course, the exception to this rule is planning music to accompany sacramental action. Just keep in mind that every seasonal song may actually have a place on specific Sundays of its season. "On Jordan's Bank" is an Advent hymn but works well on the Second or Third Sunday of Advent or even on the feast day of the Baptism of Jesus.
Thematic. Sometimes a pastor will preach on a theme and often for several consecutive weeks, so, I'll go along with them. Having regular meetings with the pastor to discuss the seasons, readings and community dynamics can be a great tool.
I also choose music based upon what the worshiping community may need to sing (we are what we sing). For instance, I once played for a church which was opposing a parole shelter or halfway house from moving in next door (who wants sinners coming to our church?) so in consultation with the pastor I scheduled "All are Welcome," “The Summons,” “Amazing Grace,” and "God has Chosen Me" for about four weeks straight. It is not enough to ask God for forgiveness if we can not forgive others. The giving of money, going to church or even serving on a committee to the church does not free people from the responsibility to forgive. The act of forgiveness is very hard, but, very easy. Despite our protestations, the shelter went in and the parolees became wonderful tenants. Not only did they attend our church, but they performed many community service projects on our building and a few of them became members, got married and started families in our congregation. We lost some of our more pious members but we can now sing "All are Welcome" and mean it, and know it, and live it. It was a true transubstantiation. Even the haters are welcome back if they are willing to forgive themselves for, adoration leaves no room for pride.
Of course, I would always first consult the Lectionary for the readings. Some liturgists use the Lectionary for what I call "Eureka Planning." That is when you read the scripture for a particular day and can match it to the text of a hymn. For instance, on the second Sunday of Easter, Years A, B AND C, the scripture reading is about Jesus appearing to Thomas and Thomas doubts that it is really Jesus so Jesus invites him to place his hand to the wound in his side. A perfect hymn or song to sing here would be "We Walk By Faith" which echoes that scene in the third verse. I may use an upbeat setting of that hymn for the opening to foreshadow what will be heard in the readings. I may use it for after the homily to augment what I know the pastor may break open in Word. The possibilities are endless. That can be the most frustrating part of planning. You can have fifteen songs which would be perfect for any one Mass but you only need four.
I have eclectic tastes and usually program music so that there is something for everybody at every liturgy crossing instrumentation and genre. During the hymns and songs, I am always cognizant of the congregation and their level of participation. If they really like a song or are moving along to it, I make note of it. If they aren't, I make note of it but then try to analyze why and then figure out how I can fix whatever may be wrong. Of course, some organists can't do this from their balcony aeries with their backs to the congregation and 54 ranks of pipes staring them in the face.
I also think that each liturgy should be a production and that each person should leave the service a different person than when they came in. That is easy to do if you can encourage them to sing one song or let out one "woot." At least on a cellular level they will have taken a deeper breath, oxygenated their blood, and they may even zap a few brain cells, leaving with a clearer mind or more energy. Singing has the power to physically change a person and for the better because it does aid in the oxygenation of the blood which does wake up the brain and that is why it is a crucial tool at the disposal of every pastoral musician. A congregation that sings, goes out into the world as better people - a transubstantiation.
If your church uses the Revised Common Lectionary, it is easy to choose music based on the scheduled readings. I would plan a tentative schedule for an entire year. If the pastor chooses the readings, I will schedule music as far as he plans but would then lean toward seasonal planning.
Just to recap, there are six criteria to consider:
A. What the congregation knows; Not the same as what you like.
B. How quickly they learn;
C. What are the needs of the assembly, congregation and outside community.
D. Seasonal songs
E. Topical and thematic songs and/or requests from the pastor
F. The Lectionary
Keep in mind that there are also dozens of websites, many are denominational or publisher based, where selections of suitable songs and hymns have already been mapped out for you. Just as a pastor can have canned homilies, your selections can be canned. Generally, if you use them, you can't go wrong. The difference is like giving someone cash for their birthday as opposed to giving them a handmade gift or something you picked out yourself. If you use a planning guide to choose your music, it will be good. If you do your homework, work with the pastor, the parish and the people, it will be better.
If worse comes to worse, there are hundreds of church musicians who post their music schedules online for their choir members and the world to see. Steal them.
When choosing music for a choir or soloist, it is pretty much the same as the aforementioned with a few added components of what is in the library, the budget and the skill level of the choir. If your choir worships music and loves to perform, well, there you have it. If the choir is in love with God, loves the people of the pew and, for them, music is not a ministry, but a tool to ministry, the sky is the limit. Adoration leaves no room for pride.
Preludes, postludes and offertories are also an expression of my faith. I try to play something spirited, dynamic and engaging. In one church I served, the pastor welcomed the people at the start of the service then he sat down and my prelude began. Every prelude had to be something interesting since they all sat there and listened intently. The postlude was the same, he invited them to sit and listen. When I was finished they were invited to go out into the world to love and serve the Lord and each other. One Sunday I played a still and quiet piece (which is rare for me) for the prelude. Because they were accustomed to toccatas, fugues and a broad range of dynamics in the prelude, a little old lady came up to me after the service with her walker and said, “What the heck was that? Don’t ever do that again.”