In my mind, the Sunday or feast day celebration is about two things, or two sides of this coin:
First, we unite ourselves as deeply as we can, body, soul and spirit, to the Blessed Trinity in the worship of Christ to the Father in the Holy Spirit. This is carried out in space and time through His Church in the Liturgy of the Hours and especially in the holy sacrifice of the Mass.
And then, believers console each other as best they can that we don't live in heaven yet, but still on earth. We search out and offer each other the best we can find by way of traces of heaven in our souls, in our communion, in God's creation, and in the poor.
And then we move back into our work with a blessed but wistful heart that this earth is not our lasting city.