According to the General Introduction to the Lectionary for Mass:
"79. In Masses assigned three readings, the three are to be used. If, however, for pastoral reasons the conference of bishops has permitted use of only two readings,
[footnote 105: See 1975 General Instruction of the Roman Missal, no. 318; Congregation for the Sacraments and Divine Worship, instruction Inaestimabile Donum, 3 April 1980, no. 1; AAS 72 (1980) 333-334.]
the choice between the two first readings is to be made in such a way as to safeguard the Church’s intent to instruct the faithful more completely in the mystery of salvation. Thus, unless the contrary is indicated in the text of the Lectionary, the reading to be chosen as the first reading is the one that is closer to the theme of the gospel, or, in accord with the intent just mentioned, that is more helpful toward a coherent catechesis over an extended period, or that preserves the semi continuous reading of some biblical book.
[footnote 106: For example: in Lent. the continuity of the Old Testament. readings corresponds to the unfolding of the history of salvation; the Sundays in Ordinary Time provide the semicontinuous reading of one of the letters of the apostles. In these cases, it is right that the priest choose one or other of the readings in a systematic way over a series of Sundays so that he may set up a coherent plan for catechesis. It is not right to read indiscriminately on one day from the Old Testament, on another from the letter of an apostle, without any orderly plan for subsequent texts.]
The 1975 General Instruction of the Roman Missal (GIRM) had:
“318. Sundays and holydays have three readings, that is, from the Old Testament, from the writings of an apostle, and from a Gospel. Thus God’s own teaching brings the Christian people to a knowledge of the continuity of the work of salvation. Accordingly, it is expected that there will be three readings, but for pastoral reasons and by decree of the conference of bishops the use of only two readings is allowed in some places. In such a case, the choice between the first two readings should be based on the norms in the Lectionary and on the intention to lead the people to a deeper knowledge of Scripture; there should never be any though of choosing a text because it is shorter or easier.”
The 2002 GIRM does not include this discussion of only two readings being allowed in some places. Instead it has:
“357. For Sundays and solemnities, three readings are assigned: that is, from a Prophet, an Apostle, and a Gospel. By these the Christian people are brought to know the continuity of the work of salvation according to the God’s wonderful plan. These readings should be followed strictly. During the Easter Season, according to the tradition of the Church, instead of the reading from the Old Testament, the reading is taken from the Acts of the Apostles.”
A justification for two readings could be, from the 2002 GIRM:
“358. … In Masses with special groups, the priest is allowed to choose texts more suited to the particular celebration, provided they are taken from the texts of an approved lectionary.”
Also from the Directory for Masses with Children:
“42. With regard to the number of readings on Sundays and feast days, the decrees of the conferences of bishops are to be observed. If three or even two readings appointed on Sundays or weekdays can be understood by children only with difficulty, it is permissable to read two or only one of them, but the gospel reading should never be omitted.”
