With Christianity out in the open, it adopted a sense of holy time. Some remark that in some cases, Christians took over pagan holidays not only for their own use but to overwhelm old observances. Likewise, with pagan architecture. Let’s read:
25. In the fourth and fifth centuries, a greater sense of the sacredness of times and places begins to emerge. Many of the local Churches, in addition to their recollection of the New Testament data concerning the dies Domini, the Easter festival and fasting (cf. Mark 2,18-22), began to reserve particular days for the celebration of Christ’s salvific mysteries (Epiphany, Christmas and Ascension), or to honor the memory of the martyrs on their dies natalis or to commemorate the passing of their Pastors on the anniversary of their dies depositionis, or to celebrate the sacraments, or to make a solemn undertaking in life. With regard to the socialization of the place in which the community is called to celebrate the divine mysteries and give praise to the Lord, it must be noted that many of these had been transformed from places of pagan worship or profane use and dedicated exclusively to divine worship. They became, often simply by their architectural arrangements, a reflection of the mystery of Christ and an image of the celebrating Church.
How significant is this? Do you suppose we would ever see a Christian takeover of Super Bowl Sunday or rock music to the exclusion of things like sex and drugs and the selling of such things? And yet it happened sixteen centuries ago.
The full document, the Directory on Popular Piety and the Liturgy, is online at the Vatican site.