If you have been granted a residence permit as a seasonal worker, you have been given a set of dates in which you can travel into Norway. This period usually lasts for 6 weeks.
Suppose you are a Ukrainian citizen who is in Norway. In that case, you can start working before the entry period if your employer offers you to start working earlier than stated. Before you start working, you must book an appointment with the police. After you have booked an appointment, you can start working for your employer.
Suppose you have worked as a seasonal worker during the last 6 months. In that case, you can not start working as a seasonal worker again until it is at least 6 months since you terminated your previous employment as a seasonal worker.