The north and the south of each have long periods of complete sunlight in complete darkness why

The earth is tilted. When the earth rotates around the sun, one side of the pole is always in darkness while the other side is in complete sunlight. For instance, when the earth is tilted toward the sun (60 degrees north for example), the northern hemisphere is experiencing the season Summer (poles have a long period of complete sunlight) whereas the southern hemisphere is experiencing Winter.