In thanksgiving to Our Lady of the Rosary.
Each time Our Lady appeared at Fatima she said, "Pray the Rosary every day."
At the first apparition, Bl. Francisco was able to see the light of the apparition but he was unable to see or hear the Blessed Virgin. Lucia told her, "Francisco cannot see you." Our Lady responded, "Let him say the Rosary, and in that way he too will see me."
Works for me!