Can you imagine seeing the Sun at midnight? Well, if you were to visit the Arctic Circle in Summer, you would see just that! Why does this happen ?
You know that the Earth's axis is titled, and because of this, one pole is inclined towards the Sun for six months of the year, and the other pole for the rest of the year. The result is that the pole facing the Sun has almost continuous daylight for those six months, while the pole facing away has almost continuous darkness during that period. In this case of the Arctic Circle, there is almost continous daylight during summer, and almost continuous darkness during winter. In fact, on june 21st, the Sun does not set at all, and can be seen even at midnight!