The Cenacle is where Jesus celebrated the Last Supper, and where the Apostles gathered on the Pentecost. It's in Jerusalem, and though the actual building is not from the time of Jesus, the location is the same one. According to Jewish tradition, King David is also buried there.
Thousands of people make pilgrimages there every year, but it's not a church, nor does it belong to Christians.
In 1523, the Otoman Empire expelled from it the Franciscan community, which had founded there its first convent in the Holy Land, two centuries before.
Since the creation of the State of Israel, the Cenacle is property of the government and is open to the public.
Mass can only be celebrated during very special occasions, like the visit of John Paul II to the Holy Land in 2000.
Following his example, Pope Francis will also presided a Mass there on Monday May 26, 2014.