Pope Francis is to mark the 100th anniversary of the mass killings of Armenians under Ottoman rule in WW1 at a church service in Rome.
Armenian President Serzh Sargsyan will be attending the ceremony which is being held to honour a 10th century Armenian mystic.
The mass deportation of Armenians in 1915 remains a highly sensitive issue.
Turkey denies Armenian claims that up to 1.5 million people were killed and that it constituted an act of genocide.
The dispute has continued to sour relations between the two countries.
Earlier, Pope Francis said that the event was an act of men who were capable of “systematically planning the annihilation of their brothers” but did not use the word genocide.
Instead he called for healing and reconciliation at a meeting last week with the visiting Armenian church delegation.
“Let us invoke divine mercy so that for the love of truth and justice, we can heal every wound and bring about concrete gestures of peace and reconciliation between two nations that are still unable to come to a reasonable consensus on this sad event,” he said.
On Sunday, Pope Francis will also declare the mystic St Gregory of Narek a doctor of the church. Only 35 people have been given the title, reports the Associated Press news agency.
Armenia marks the date of 24 April 1915 as the start of the mass killings. The country has long campaigned for greater recognition of what it regards as a genocide.
In 2014, Turkish PM Recep Tayyip Erdogan offered condolences to the grandchildren of all the Armenians who lost their lives for the first time.
But he also said that it was inadmissible for Armenia to turn the issue “into a matter of political conflict”.
Armenia says up to 1.5 million people died in 1915-16 as the Ottoman empire split. Turkey has said the number of deaths was much smaller.
Most non-Turkish scholars of the events regard them as genocide. Among the other states which formally recognise them as genocide are Argentina, Belgium, Canada, France, Italy, Russia and Uruguay.
Turkey maintains that many of the dead were killed in clashes during World War I, and that ethnic Turks also suffered in the conflict.