For the first time in nearly 1,000 years bone fragments of Saint Nicholas are being moved from their Italian resting place, to be worshipped in Russia.
The 4th-Century saint is one of the most revered figures in the Russian Orthodox Church. After his death, Italian merchants brought his body from Myra, in modern-day Turkey, to Italy.
Some fragments of his ribs are kept in Bari, southern Italy. They are being flown on a specially chartered plane to Moscow on Sunday.
"This is an unprecedented event," said Alexander Volkov, a representative of the Moscow Patriarchate. "These relics have never before left Italy."
The relics - on loan to Russia until late July - will be moved from Bari's Basilica of St Nicholas to the Cathedral of Christ the Saviour in Moscow. There they will be blessed by the head of the Russian Orthodox Church, Patriarch Kirill, in a ceremony on Sunday evening.
Thousands of Orthodox believers are expected to visit the relics. They will also be taken to the Alexander Nevsky Monastery in St Petersburg.
Such relics have an important role in the Orthodox faith. In 2011, part of what was believed to be the Virgin Mary's belt went on display in Moscow. Church-goers were content to queue for up to 24 hours - and the line snaked back for several miles next to the Moskva River.
Permission to lend the bones of St Nicholas to Russia came at an historic meeting between Pope Francis and Patriarch Kirill, in Cuba in 2016.
It was the first such meeting since their two branches of Christianity split nearly 1,000 years ago.
The Moscow Patriarchate says Russian jewellers have created a special "ark" to transport the bone fragments to Moscow. They have lain undisturbed in Bari since 1087.
It is not yet known whether Russian President Vladimir Putin, a keen church-goer, will attend the ceremony. In 2011 he met the plane carrying the belt of the Virgin Mary when it touched down in St Petersburg.
Moscow authorities expect Russians from all over the country to visit the relics, and say they will provide water to those standing in the queue.
Churches across Moscow will ring their bells at 18:00 (15:00 GMT) on Sunday to mark the relics' arrival.
From 22 May to 12 July the relics will be on show at Moscow's Cathedral of Christ the Saviour. The relics will be in St Petersburg in the period 13-28 July.
Who is St Nicholas?
A favourite saint among Russians - thousands of Russian churches are named after him
Patron saint of sailors, merchants, archers, repentant thieves, children, brewers, pawnbrokers and students
His habit of bestowing gifts, often in secret, led to him being immortalised as Santa Claus or Father Christmas
Preached in Asia Minor (now Turkey) and was originally buried in Myra
In 11th Century rival merchants from Venice and Bari raided his grave and took the bones back to Italy
Some of his remains were also buried in Venice, where they are kept to this day.