The celebration culminates on August 14 and 15 with the Way of the Cross and the main mass of the celebration. However it started on August 11 with services and prayer.
Latvian Radio saw the first groups of pilgrims arrive, like the two teenagers Laura and Ingūna, who were seen resting by the white basilica, tired, but still singing as is customary on the walk to Aglona.
While Ainārs and his girlfriend hitchhiked from Tallinn, having walked part of the way on foot, in what has become an annual tradition for them.
Latvia's highest officials - President Raimonds Vējonis, Prime Minister Māris Kučinskis and parliament speaker Ināra Mūrniece - are also to take part in the celebrations on Monday.
For locals the celebration is associated with crowds, when there's a huge queue at the store at Aglona, the town in Latvia's east where the celebration is held.
However for some locals, like one Jānis, it's extra income as he can earn some money helping to set up tents.
The peddlers' street also becomes crowded each year, with sellers from Lithuania and Kurzeme having set up their tents first.
All the while road-weary pilgrims can be seen collecting cool water at the nearby holy spring, which is believed to have healing properties.
After spending Saturday night in prayer, on Sunday the sacred square of the basilica saw the Holy Mass being held, followed by a vesper and singing spiritual music.
On Sunday evening the traditional Way of the Cross was held, followed by a midnight service.
While on Monday the main Holy Mass is taking place from 12 in the afternoon, with archbishop and Catholic diplomat Pedro López Quintana addressing those present.
The ceremony is also being broadcast on Latvian national radio and television.