Artic Circle is the cutoff point.
If your standing on the Artic Circle (66 degrees, 32 minutes N) on the Summer Solistice, the sun will drop to the horizon at midnight & rise back up again. Further north then that, you will have more and more days before & after the summer solistice where the sun never sets.
On the opposite side of the calender, at the winter solistice, the sun will just come up to the horizon, then go back down, from a vantage point on the arctic circle.
Note that 90 - 66.5 = 23.5 = the latitude of the Tropic of Cancer = the angle between the earth's pole and it's orbital plane.