This Feastday was instituted originally to honour the martyrs, as the early Christians developed the practice of solemnising the anniversary of a martyr's death at the place of his/her martyrdom. In the fourth century Dioceses close to one another began to interchange feasts, and as time went on the number of martyrs increased to the point where a separate day could not be set aside for each of them.
The Church, feeling that every martyr should be venerated, set a common day for all. Pope Boniface IV consecrated the Pantheon in Rome to the Blessed Virgin and all the martyrs, on the 13th. May 609 or 610.
The Pantheon is a magnificent building dating from the first century, and its dome was so impressive that when Michelangelo was designing the dome on St Peter's Basilica, some fourteen centuries later, he made it slightly smaller that that of the Pantheon, in deference to its antiquity and beauty of design and unknown method of construction.
I read somewhere that only in very recent years, with all our advanced technology and building methods, could engineers now construct a dome of equivalent size.
I believe it is some 360 feet in diameter. It has a large hole in the centre, through which rain and snow pour at times, and the floor is sloped towards the centre with a drain hole to let the water out. A magnificent building.