Crowds will be aided by a choir of 2,000 at Cofton Park and 800 at Bellahouston and there will be “detailed and focused” rehearsal before the Masses start.
The choirs will be accompanied by brass and timpani on the day but, according to Mr MacMillan, any parish can perform the setting as long as it has an organ.
Mr MacMillan said he tried to make the basic melody simple so that congregations would pick it up easily. “It’s not a lot of time to bed the music down in dioceses and parishes,” he said.
He also said he hoped it would be “appropriate to the text and the way the drama of the Mass unfolds”.
Mr MacMillan said: “There has to be a sense of awe at the words of ‘Blessed is he who comes in the name of the Lord’, just before the consecration. And the Gloria is a huge raising of hearts to heaven, a great joyous outburst from the very early days of the Church, that again has to have a very different flavour.”
Mr MacMillan admitted he was apprehensive about the setting being sung “in the middle of a field”.
“Singing out of a field is tricky – it’s just a very strange experience standing in the middle of the field and being expected to sing. And Catholics are reluctant singers at the best of times.
“I just hope that people rise to the challenge. At first encounter it might feel strange, but if they have the text and music with them I hope they will really join in on the day,” Mr MacMillan said.