This year's Lucia is Sara Ray, a 24 year-old education student from Esse in Ostrobothnia.
Saint Lucia Day has Catholic and Orthodox origins and is celebrated across Finland, especially in Swedish-speaking communities.
Each year's Lucia is chosen by public vote as a charity fund-raising scheme, and then works for several weeks visiting hospitals, retirement homes, schools and other facilities.
On Friday, Sara Ray will be crowned at 5pm in a ceremony in Helsinki's Lutheran Cathedral by former Finnish President Tarja Halonen.
She will emerge from the church and descend to Senate Square. She and her entourage will then lead a procession through the city, as tens of thousands of people gather in the square for the spectacle.
Many towns and cities chose their own local Lucia, and the day is celebrated in many child daycare centres and schools, especially in those where Swedish is the main language of instruction.