The French president Emmanuel Macron will lead mourners marking the one year anniversary of the murder by Islamist militants of the priest Fr Jacques Hamel.
A monument will be unveiled and Mass will be said and televised in the same church of St-Étienne-du-Rouvray in Rouen, Normandy where Hamel was slain last year.
The French Interior Minister Gérard Collomb will also be attending, alongside Anouar Kbibech, the former president of the French Council of Muslim Worship.
On July 26, 2016, two 19-year-olds who claimed allegiance to Islamic State stormed the altar during morning Mass. Witnesses said the murder was filmed by the killers and Islamic rites were recited. Two nuns were held hostage as the terrorists brought Hamel to his knees before they slit his throat. One of the nuns was able to escape and call the police, who arrived quickly and shot the attackers dead as they fled the church shouting 'Daesh' (another name for ISIS).
The anniversary Mass will be a rare display of Church and state unity in a country often defined by fierce secularism.
This evening, Vespers will be held at the Basilique Notre-Dame de Bonsecours in Rouen, followed by prayers at Hamel's tomb.
Archbishop Dominique Lebrun of Rouen has triggered the formal process of sainthood for Hamel to be officially declared a martyr. Statements will be heard from members of the congregation who witnessed his death, along with the priest's family members, clergy and Muslim friends.
Pope Francis has waived the traditional five-year period before the cause for sainthood could normally commence.
In April, a special Mass was held by the Pope in the Basilica of St. Bartholomew on Rome's Tiber Island, where a holy book of Hamel's is on display near relics of Spanish and Italian priests.
During that Mass, Hamel's sister, Roseline, said that while he had been fragile, 'he was also strong – strong in his faith in Christ, strong in his love for the Gospel and for people, whoever it was, and, I am certain, also for his killers'.
Hamel was born on November 30, 1930 in Darnétal, France, and ordained on June 30, 1958.
Before becoming a priest, Hamel was a soldier in Algeria for 18 months, refusing to become an officer because he did not want to issue orders that would lead to deaths.