Why does this happen? The Earth is tilted. As it spins around the sun once a year, the top of the world (the North Pole) ends up leaning towards the sun in our summer and away from the sun in our winter. The further north you go, the bigger that effect gets.
In a town called Kiruna, in the far north of Sweden, the sun stays up for around 50 days in summer without ever setting. People play football at midnight in full sunshine. To sleep, they pull thick black curtains across the windows so the bedroom feels like night.
In December, the opposite happens. The sun doesn't rise at all for about a month. It isn't pitch black - the sky still goes light grey around midday - but the sun stays hidden below the horizon. Children walk to school in the dark and home in the dark. Streetlights stay on all day.
There is one big bonus to the dark winter: the Northern Lights. These are huge curtains of green, pink and purple light that dance across the night sky. They happen when tiny particles from the sun smash into the air high above the Earth. People travel from all over the world to see them.