Easter Sundays (of the season) are organized thus:
100. The Gospel readings for the first three Sundays recount the appearances of the risen Christ. The readings about the Good Shepherd are assigned to the Fourth Sunday. On the Fifth, Sixth, and Seventh Sundays, there are excerpts from the Lord’s discourse and prayer at the end of the Last Supper.
The first reading is from the Acts of the Apostles, in a three-year cycle of parallel and progressive selections: material is presented on the life of the early Church, its witness, and its growth.
For the reading from the Apostles, the First Letter of Peter is in Year A, the First Letter of John in Year B, the Book of Revelation in Year C. These are the texts that seem to fit in especially well with the spirit of joyous faith and sure hope proper to this season.
The plan for the Easter octave, weekdays of the second week and beyond, as you know, gives us a strong dose of Acts. The complementary texts of John’s Gospel, too:
101. As on the Sundays, the first reading is a semicontinuous reading from the Acts of the Apostles. The Gospel readings during the Easter octave are accounts of the Lord’s appearances. After that there is a semicontinuous reading of the Gospel of John, but with texts that have a paschal character, in order to complete the reading from John during Lent. This paschal reading is made up in large part of the Lord’s discourse and prayer at the end of the Last Supper.
You may know that the 1998 Lectionary provides particular second readings for the feasts of Ascension and Pentecost for cycles B and C
102. For the first reading the Solemnity of the Ascension retains the account of the Ascension according to the Acts of the Apostles. This text is complemented by the second reading from the Apostle on Christ in exaltation at the right hand of the Father. For the Gospel reading, each of the three Years has its own text in accord with the differences in the Synoptic Gospels.
In the evening Mass celebrated on the Vigil of Pentecost four Old Testament texts are provided; any one of them may be used, in order to bring out the many aspects of Pentecost. The reading from the Apostles shows the actual working of the Holy Spirit in the Church. The Gospel reading recalls the promise of the Spirit made by Christ before his own glorification.
For the Mass on Pentecost day itself, in accord with received usage, the account in the Acts of the Apostles of the great occurrence on Pentecost day is taken as the first reading. The texts from the Apostle Paul bring out the effect of the action of the Spirit in the life of the Church. The Gospel reading is a remembrance of Jesus bestowing his Spirit on the disciples on the evening of Easter day; other optional texts describe the action of the Spirit on the disciples and on the Church.
Pentecost Vigil … too bad we can’t use all four. A liturgy scholar I knew once told me that was the ancient intention, but I don’t have a reference for that bit.