Since you are coming to Finland for seasonal work, you will need either a visa, a certificate or a residence permit for this purpose. Which of these permits you need depends on how long you will be working and from which country you are coming to Finland.
If you are going to work for less than three months, you will need a certificate for seasonal work or a visa for seasonal work.
If you are going to work for three to six months, you will need a residence permit for seasonal work.
Finnish Immigration Service