The Northern Lights are present year-round, but may only be observed when it is dark in Northern Norway, and that’s from the end of August till mid of April.
During the period from September till the end of March, we have plenty of hours of darkness. There is no set time to see the Aurora as the geomagnetic disturbance in the earth magnetic field is caused by solar flares, and they can occur any time! It’s best to go outside as soon as it’s dark and the sky is starry and stay out watching the sky as late as past midnight.
From mid-April, the amount of darkness each day reduces sharply, and from May 21 you can see the Midnight Sun in Tromsø. The contrasts are enormous in the Arctic. During summer above the Arctic Circle, the sun never sets, and it is light around the clock.