The Saint-Etienne-du-Rouvray church, in the small town of some 27,000 near the city of Rouen, was to pay tribute to Father Jacques Hamel, the 85-year-old priest whose throat was cut at the foot of the altar on 26 July in an attack claimed by the Islamic State (IS) armed group.
The perpetrators, Adel Kermiche and Abdel-Malik Petitjean who were both 19, were shot dead by police following a siege.
Guy Coponet, an elderly parishioner who was forced to film the priest's murder, stabbed three times and left for dead, told local paper Normandie actu that he would attend the ceremony, which will be preceded by a procession.
Only 250 people, who have received invitations, will be allowed into the small church but the proceedings will be broadcast on a giant screen to those outside.
Muslims to join procession
Local mosques have appealed to Muslims to turn out in solidarity against IS.
The mass and ritual cleansing, which will seek God's forgiveness for the profaning of the church, will be presided over by over by Archbishop of Rouen Dominique Lebrun, who celebrated a funeral mass in August for the slain priest at Rouen Cathedral with President François Hollande and Prime Minister Manuel Valls in attendance.
An investigation is under way to find out if Kermiche and Petitjean had accomplices.
The pair telephoned someone to report what they were doing during the attack, according to Coponet's lawyer Mehana Mouhou.