Certainly, the Ascension of the Lord is a major feast - and it is worthy of the additional attention it gets by pre-empting the Seventh Sunday of Easter. However, what we lose is also something important: our opportunity to hear Jesus' final discourse and his prayer for the unity of his church:
Lifting up his eyes to heaven, Jesus prayed saying:Jesus prays for his believers - and says that the world will understand the Father better if believers are "one". Over and over he says the word "one".... emphasizing the importance of Christian unity to the coherence of the message of the Gospel.
“Holy Father, I pray not only for them,
but also for those who will believe in me through their word,
so that they may all be one,
as you, Father, are in me and I in you,
that they also may be in us,
that the world may believe that you sent me.
And I have given them the glory you gave me,
so that they may be one, as we are one,
I in them and you in me,
that they may be brought to perfection as one,
that the world may know that you sent me,
and that you loved them even as you loved me.
Father, they are your gift to me.
I wish that where I am they also may be with me,
that they may see my glory that you gave me,
because you loved me before the foundation of the world.
Righteous Father, the world also does not know you,
but I know you, and they know that you sent me.
I made known to them your name and I will make it known,
that the love with which you loved me
may be in them and I in them.” (John 17:20-26)
This is the reading from which Blessed John Paul II drew the title of his great document on ecumenism: Ut Unum Sint ("That They May be One"). It is an important reading because it provides the biblical basis for Catholic ecumenical dialogues with other Christians. It is quoted in the Catechism of the Catholic Church in paragraph 820 as the basis for an understanding that the Church is ONE.
Recently, I gave a talk on Catholic teaching on ecumenism at a regional parish event in an adjacent diocese. A number of people in the audience confided that before my talk, they had never even heard of the word "ecumenism". It is true that the subject has gone out of fashion in many dioceses. All the sadder that we lose the natural opportunity to hear preaching on the necessity of Christian unity for effective evangelization when this gospel is replaced by the one for Ascension. When cultural practice (not attending Mass on weekday Holy Days of Obligation) necessitates a change, but we are far away from a time when the Lectionary will be revised to reflect that, we have a necessary loss... and sadly, an important one.