The feast of the Ascension commemorates and celebrates the close of the series of appearances, which Jesus Christ made to the disciples after his Resurrection.
Jesus's final apparition ends with the irreversible entry of his humanity into divine glory, symbolised by the cloud and by heaven, where he is seated from that time forward at God's right hand
- as the Catechism of the Catholic Church puts it (CCC659). Something, if you like, came to an end.
Secondly, as the Preface at Mass as well as the Catechism both emphasise, the feast celebrates the fact that Jesus Christ,
as the head of the Church, precedes us into the Father's glorious kingdom so that we, the members of his Body, may live in the hope of one day being with him for ever. (CCC 666)
Thirdly, 'having entered the sanctuary of heaven once and for all', the risen Lord Jesus
intercedes constantly for us as the mediator who assures us of the permanent outpouring of the Holy Spirit. (CCC 667)
As 'high priest of the good things to come' (Heb 9: 11), the risen Christ is
the principal actor of the liturgy that honours the Father in heaven (CCC 662).
We have 'a high priest' - 'one who is seated at the right hand of the throne of the Majesty in heaven', always making intercession for those who drawn hear to God through him (Heb 8:1; 7:25).
Fourthly, as the imagery of 'being seated at the Father's right hand' signifies, the risen Christ's Ascension is 'the inauguration of the Messiah's kingdom', the fulfilment of the prophet Daniel's vision:
To him was given dominion and glory and kingdom, that all peoples, nations, and languages should serve him; his dominion is an everlasting dominion, which shall not pass away, and his kingdom one that shall not be destroyed. (Dan 7:14; cf. CCC 664)
Each of these four different aspects of the Ascension needs to be taken into account as we celebrate the feast and contemplate the implications. The risen Christ ceased appearing to his disciples; he is hidden from our eyes. That is just a fact.
Secondly, since we are incorporated into him by faith and baptism we are members of his Body, which means that where he has gone we shall also follow. That is a promise.
Thirdly, the central metaphor for heaven, for being for ever in the presence of God, comes from the analogy of liturgical worship: the risen Christ is the 'principal celebrant', so to speak, of the unceasing worship in which God is praised and intercession made for us sinners. That is not so much a promise as an attempt to put into words what being with God for ever must be like; it is only a picture, of course, certainly not the only one in the repertoire of biblical images, and perhaps not the most plausible and attractive analogy for many people.
The picture of the ascended Christ 'seated at the right hand of the Father' may seem to many people even more unattractive. Clearly it derives from the many allusions in Scripture to scenes of courtroom drama. It is little exaggeration to say that the principal metaphor in the Bible for the relationship of God to us is as our judge. Being 'seated at the right hand of the Father' means that the authority to pass judgement is now in Christ's hands --- as the next clause of the Creed of course says:
He will come again to judge the living and the dead.
The Ascension means his being seated on the judicial bench, so to speak; it implies his being enthroned as Judge.
Putting it less metaphorically, we live 'under the law of Christ (e.g. 1 Cor 9:21):
Jesus is Lord!
Less metaphorically still, he is the one to whom we are accountable: which is why the feast of the Ascension sends us back once again to let the light shine in our hearts 'of the knowledge of the glory of God in the face of Christ' (2 Cor 4:6).