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Pope Francis is ushering in the most solemn period in the Catholic liturgical calendar by celebrating a Holy Thursday Mass, made more poignant this year following the devastation of Paris' Notre Dame Cathedral.

The Crism Mass celebrates the unity of priests with their bishops. During Thursday's service, Francis blesses the holy oils that will be used over the course of the year in the administration of various sacraments.

In his homily, Francis stressed that when priests use the oil, they are distributing their vocation and heart to the people of God.

This year, Holy Week — which for Catholics commemorates Christ's crucifixion, death and resurrection — has taken on particular meaning following the fire at Notre Dame, a symbol of French Catholicism.

Francis has offered his condolences repeatedly to the French faithful.