Along with the Roman Missal, the other book necessary for the proper celebration of Holy Mass is the Lectionary. The recent translation changes that began with Advent this year affect only the Missal, not the Lectionary. But the Lectionary, also, can cause its share of confusion when some of its fundamentals aren’t understood.
For starters, the readings are in two major cycles. The first cycle consists of readings for Sunday Mass, broken into Years A, B, & C. The second cycle is for weekday readings, and they are classified as Years I & II. In addition to these two major cycles, there is a section of Common readings provided for various celebrations and feast days (e.g., dedication of a church, feasts of martyrs, etc.), as well as a volume of readings for ritual Masses (e.g., weddings, funerals, confirmation, etc.).
So, which readings get read when? It might be helpful first to understand the structure of the different types of liturgical celebrations:
1. Solemnities: all Sundays & celebrations of major mysteries of faith and occasions of highest local importance (e.g., Christ the King, Immaculate Conception, feast of parish patron, etc.)
2. Feasts: celebrations of Our Lord, Our Lady, or the saints that are of particular importance, but not such importance as solemnities (e.g., Transfiguration, Our Lady of Guadalupe, St. Lawrence, etc.)
3. Memorials: celebrations of the saints that do not rise to the significance of feasts or solemnities (e.g., St. Isaac Jogues, St. Rose of Lima, St. John Neumann); the celebration of some memorials is obligatory, while for others it is optional
4. Ferial Days: celebrations in which no specific mystery, title, or saint is celebrated (e.g., Thursday of the 24th Week in Ordinary Time, Tuesday of the 3rd Week of Advent)
By and large, the readings for Sunday Masses and other solemnities are “proper” and not to be replaced by other readings. Exceptions to this are very rare. Readings for feasts, similarly, are often proper. On the contrary, very few memorials have proper readings (e.g., the Gospel of Martha & Mary on the memorial of St. Martha, the Gospel of the crucifixion on the memorial of Our Lady of Sorrows, etc.). It is permissible to use readings from the Common of Martyrs/Pastors/Doctors/Virgins/Holy Men on memorials, but most priests will not choose to do so.
Instead, most priests will use the readings from the daily cycle (either year I or II, appropriately) for memorials and ferial days. The primary reason for this is that the daily cycle contains continuous readings of books of the Bible. The fewer interruptions to this continuous reading, most liturgists would say, the better and more cohesive the liturgy becomes.
So, with weekday readings, there is a greater degree of latitude. A priest could choose to use special readings for a votive Mass, or the readings of the day, or readings from the Common appropriate to a particular saint. At all times, these choices are to be made not simply out of deference to the convenience or devotion of the priest, but in favor of the good of the people of God.
If you find the Lectionary & and the choice of readings for Mass confusing, you are not alone. But we can be confident in our mother, the Church, for she always provides us with an abundance of rich nourishment through the Word of God.