Families and friends will join Spanish royalty and politicians to gather in the Gaudi-designed cathedral at 6pm for the memorial service, to remember the 150 victims of the crash, which happened in the southern French Alps.
Spain's King Felipe and Queen Letizia will be in attendance, as will Spanish Prime Minister Mariano Rajoy and the regional President of Catalonia, Artur Mas.
Over 1,200 people are set to attend the service, while families of the victims will first attend a "brief and modest" reception in the Catalan parliament, where they will be greeted by Artur Mas and President of the parliament, Nuría de Gispert.
The ceremony will take part in the Catholic cathedral because of the “deeply rooted” Catholic tradition in Catalonia, according to sources close to the Archbishop of Catalonia.
Lluís Martínez Sistach, the current Archbishop of Catalonia will conduct a Catholic mass, but at the request of some of the relatives of victims interfaith aspects will be included.
Representatives from the Evangelical church, as well as the Jewish and Muslim communities will say a few words.
The four religious leaders will then say a prayer in memory of the victims.
"It will be a prayer for the dead, whatever religion they were," sources told Spanish daily, 20 minutos.
The ceremony will be multilingual, conducted in Catalan, Spanish, French, German, English and Greek across different prayers and songs.
All 150 victims, including 50 Spaniards, were killed instantly.
Hundreds of family members gathered in Cologne on April 17th for a memorial service in the German city's cathedral.
The Sagrada Familia, designed by Antoni Gaudi is still under construction, but was consecrated by Pope Benedict XVI in 2010.