The refinery will be in the offline mode during the maintenance program, which will involve maintenance on around 2,000 pieces of equipment.
It will also include replacing various process furnaces and other equipment.
Neste Oil refinery manager Elina Herrala said the company expects to log a total of around 360,000 man-hours on the project and the turnaround will be a major effort.
"The nearly 1,000 people that we will be bringing on site will also be reflected in the nearby communities of Raisio and Naantali and should increase demand for a range of services."
The company said that the refinery will be back online by June, this year.