In post #5 it was asked “So, if I wander into Mass, there are two options (typically) for readings I could be hearing - ferial readings or “feast day saint” readings? Do I have that right?”
Usually more options are possible than these two.
The 1981 General Introduction to the Lectionary for Mass has:
“63. Pastors may wish to respond specifically from the word of God to the concerns of their own congregations. Although they must be mindful that they are above all to be the heralds of the entire mystery of Christ and the Gospel, they may rightfully use the options provided in the Order of Readings for Mass. This applies particularly to the celebration of a ritual or votive Mass, a Mass in honour of the saints, or one of the Masses for various needs and occasions. In light of the general norms, specific faculties are granted for the readings in Masses with particular groups.”
A footnote to this refers to three documents, including the 1973 Directory Masses with Children, nos. 41-47. This includes: “43. If all the readings assigned to the day seem to be unsuited to the capacity of the children, it is permissible to choose readings or a reading either from the Lectionary for Mass or directly from the Bible, but taking into account the liturgical seasons.”
In n. 82 of the General Introduction to the Lectionary for Mass it has under the heading “Weekday readings”
“The one using the Order of Readings for weekdays must check to see whether one reading or another from the same biblical book will have to be omitted because of some celebration occurring during the week. With the plan of readings for the entire week in mind, the priest in that case arranges to omit the less significant selections or suitably combines them with other readings, if they contribute to an integral view of a particular theme.”
The psalm need not be from the Lectionary, but may use those selected in two other books. From the 2012 General Instruction of the Roman Missal approved for Australia: "61. … Instead of the Psalm assigned in the Lectionary, there may be sung either the Responsorial Gradual form the Graduale Romanum, or the Responsorial Psalm or the Alleluia Psalm from the Graduale Simplex, as described in these books.