One of the greatest buildings in the world that I have visited is the Pantheon in Rome. Today it is its 1400th birthday as a Christian church although it dates from the 2nd century AD and parts of the building to the reign of the Emperor Augustus. Im not sure how many times a week they celebrate Mass there but it seems to be on a regular basis.
The great thing about this video is that you get to see parts of the building that you dont get to see as a tourist. Notice all the original Roman brickwork.