They are the "rock star cardinals". And their "gigs" are being kept strictly secret to make sure they don't get the rock star treatment from over-excited fans when they descend on Poland later this month.

About 70 "red hatters" are among 1000 bishops who are joining Pope Francis at World Youth Day in Krakow at the end of July.

There are 600,000 registered pilgrims from around the world but numbers are expected to swell to as many as two million by the final Sunday Mass at the festival, which happens about every three years and is known as "Glastonbury with God".

World Youth Day is regarded as a place for young people to explore vocations to the priesthood or religious life - or to find a future spouse among the thousands of like-minded young people. Faith, family, justice and mercy are among the themes this year. In particular, youngsters find a powerful sense of community spending nearly a week with other young Catholics. 

One of the biggest miracles is the catering operation. Jesus just had to feed 5000. The volunteers in Krakow will have to feed a daily lunch queue of 600,000. 

For security reasons, Poland has been allowed to suspend the Schengen freedom-of-movement agreement until 2 August, because of World Youth Day and an earlier NATO summit. It will be the largest single gathering of bishops alone since the Second Vatican Council in the 1960s.

The cardinals and bishops will be delivering the catechesis "masterclasses" in the 500 parishes around Krakow which have volunteered to host pilgrims. Some are particularly well-known and attract thousands when they speak.

To foil any tendency among their young Catholic fans to mob individual bishop by using Snapchat and Instagram to alert friends where their favourites are speaking, the individual catechesis dates are being withheld and the bishops will journey anonymously on buses to their destinations.

Father Christopher Jamison, the Benedictine monk who is former Abbot of Worth Abbey, and who is among those going to Krakow with 3000 others from the Westminster Archdiocese, said: "People everywhere loves festivals. Glastonbury has been and gone. World Youth Day is up there with the best of them. Some people describe it as Glastonbury with God."

He said it was particularly significant for the Church because of research showing that so few men aged 18-25 attend Mass that this demographic no longer registers at all in surveys. "There are men going to church but not enough to register in the figures. Yet just under half of pilgrims to World Youth Day are from this group."

A new aspect this year is sport. "I really don't know how they are going to organise a minor version of the World Cup," said Father Christopher. 

Father Pawel Rytel, of the Bishops' Conference of Poland said: "There is an atmosphere of joy in Poland." The last time the festival was held in Poland was 25 years ago when it was mainly people from eastern Europe. "This time it will be young people from all over the world."

He added: "Certain of the bishops are a bit like rock stars. The problem is they don't want everyone to rush off to any particular venue and make it completely overcrowded, so we are putting them on buses and not telling anyone where they are going."

James Kelliher, 27, coordinator for Westminster Youth Ministry, who helped organise the festival in Brazil three years ago, met his wife Bruna there. They kept their relationship going by social media and married ten months ago. "For me, World Youth Day is going to have a lifelong legacy," he said.

During his time in Poland, Pope Francis and many of the other bishops will visit the Auschwitz-Birkenau death camp. Six million Polish people were killed during the war, of which three million were Jewish.