Arguably, the sun is the most essential thing for life on this planet. So, it comes as no surprise that ancient cultures either worshiped the sun directly or made sun gods key members of their pantheons. The Egyptians had Ra. Apollo was worshiped across the Mediterranean. Later, Roman rulers tried to make Sol Invictus (the Invincible Sun) the official god of the Empire. So, it is no surprise that the Romans named the first day of the week Dies Solis (Sun Day). However, in later Romance languages (like Spanish, French, or Italian), the name changed to reflect Catholic culture (i.e., domingo/domenica/dimanche).
The Vikings seem to have had a different view, though. In Norse belief, the sun was not some powerful masculine god. Instead, the sun was a woman (a goddess presumably, but never expressly called that). This woman, Sunna or Sól, drives her chariot lit from the sparks of Muspellsheim (the land of fire) across the sky (i.e. Sunna’s Day or Sun Day). Sól’s horses are called Arvak and Alsvinn, and they are cooled by bellows under their shoulders. Sól is continuously pursued by a giant wolf named Skoll, who is a relative of Fenrir.
There is significant archeological evidence that there was once a prominent cult of the sun in Scandinavia. However, almost all of these monuments and solar symbols faded away in the 6th century. This coincides with a great time of famine, frost, and darkness in the north that decimated the population and shattered people’s way of life. This little ice age was caused by two volcanic eruptions around the 540s. Some archaeologists theorize these catastrophes discredited the sun cult, driving the survivors of those grim decades to alter their faith. So, by the time of the Vikings some three centuries later, the Norse had no significant sun deity – just a woman fleeing from wolves. Many of the fertility and prosperity features one would expect from a sun deity were (apparently) transferred to gods and goddesses like Freyr or Sif.