Nuns from a closed order will be venturing outside the convent for only the third time on Sunday to join in one of the largest outdoor Masses to be held in Wales for years.The service at Cymer Abbey, Dolgellau, is part of a year of celebrations of the life of local saint,
"Nine Carmelite nuns from Dolgellau are coming. This is only the third time they've ever been allowed to leave the convent," said Sue Roberts of the Catholic Circle, whose idea it was to hold the Mass.
"Cistercian Monks from the enclosed abbey on Caldey Island are coming too, as well as Benedictine monks from Belmont and Downside Abbey. This is the order St John belonged to."
Pope Benedict XVI has sent his blessings for the service.
"This promises to be a very moving and emotional tribute," said Archbishop of Cardiff, the Most Rev Peter Smith.
"It's a sad but undeniable truism that a genuine prophet is hardly ever honoured in his own country, but I'm hoping that this magnificent Mass will help make St John Roberts as famous in his native Wales as he is in countries like France and Spain."
St John Roberts was born at Rhiw Goch farm near Trawsfynydd and it is believed that he received his early education from a monk at Cymer Abbey before travelling to France and converting to Catholicism.
He trained as a priest in Spain, before returning to work with the poor in London. After being repeatedly exiled to France by the anti-Catholic authorities, he was finally sentenced at Tyburn on 10 December, 1610. He was canonised in 1970.
"This gathering is the biggest they've had at Cymer Abbey in a long time, if not ever," Sue Roberts said.
"People from the Senedd, Parliament and representatives of lots of Christian denominations from throughout Wales have accepted invitations to come."
Sue is particularly looking forward to the procession of monks and priests into Cymer Abbey, accompanied by three local choirs singing plainsong.
"The singing will be in Latin, and it's what St John would have been familiar with over 400 years ago," she explained. "It will be wonderful, particularly on 6 June, the important feast day of Corpus Christi - the body of Christ."
After the Mass Sue will be turning her attention to a pilgrimage from Cymer Abbey to Tyburn from 15 to 18 July and the first-ever bilingual service at Westminster Cathedral, the seat of Catholicism in Britain.
"Archbishops Rowan Williams and Vincent Nichols will be holding a joint service there for the first time ever," she said.
"But the person who will steal the show is Mari Fflur, the 13-year-old singer from Llandwrog who will sing a specially-commissioned poem by chaired bard Dafydd Pritchard, dedicated to St John Roberts."
Sue is also interested in visiting the crypt at Tyburn, where relics including John Roberts' fingerbone can be viewed.
The Cymer Abbery service is open to all and buses will be laid on from throughout the diocese.
Those arriving by car are asked to park at the Marian in Dolgellau town centre and catch a free shuttle bus to Cymer Abbey.
SIC: BBC