But astronomical properties have directly shaped the culture.

The reason why normal activity slowed down around this time is astronomical. In Stockholm today, daylight started at around 9 in the morning and it was dark before 3 in the afternoon. Of every 24 hours, more than 18 are spent in darkness. Even daylight hours come with an asterisk: usually it's just a slightly less dark version of the night, because daytime rarely means sunshine. Stockholm sees less than an hour of sunshine per day in December, mostly clumped together on a few sunny days that you can easily miss if you're busy indoors.

The rich baggage of folk traditions and mythology is also deeply related to the shortest/longest days of the year - rebirth of the Sun, victory of light over darkness, etc are clearly of astronomical origins, with folk stories adding a layer of mystique to stimulate imagination and make things more interesting.

Midsummer, on the other side of the year, forms a pair with winter solstice, and is as big holiday as Christmas. In rural communities, Midsummer marked a brief pause between planting and harvesting. In modern times, Midsummer marks the start of summer vacations. The country shuts down in late June and your emails won't get replies before August as people are enjoying the 18 hours of sunshine of Nordic summers. :)

In few languages I know, name is about the night, which is pretty astronomical thing. In Scandinavian countries they use pagan name while celebrating Christian holiday, so cultural thing is pretty loose here.