Winter sacrifice? You’re doing it wrong. There is a reason that Wikipedia isn’t an accepted source for academia – it is very capable of being wrong.
There are four main available works and many interpretive Norse guides based on those works. Those works include The Poetic Edda (10th – 13th century), The Prose Edda recorded by Snorri Sturluson (approx. 1220), The Chronicle of the Kings of Norway by Snorri Sturluson (approx. 1230), and Saxo Grammaticus’s The History of the Danes (approx. 1185).
After spending many hours reading through these, of the few references to Ullr, The Prose Edda contains the most. He is a great hunter, the fastest on snowshoes (or skis). According to the literature, it was essential to call on him for a good hunt or combat. He was recognized as quite an important god but never given a proper “god of …”.
The literature is littered with references to Skadi. A giant by nature and a goddess by marriage, her status is significantly more apparent. Her marriage to Njord (god of the sea) is told in all four works. She was pissed when her father was killed, so Odin let her choose a husband from the gods by the appearance of only their feet. After nine days at the sea, she despised the ocean and headed back to the snowy mountains. She and Njord parted ways, and she continued to be influential in references to hunting and winter.
“Sleep could I never | on the sea-beds,
For the wailing of waterfowl;
He wakens me, | who comes from the deep—
The sea-mew every morn.”
In The Chronicle of the Kings of Norway, the biggest game-changer occurs. The story of Odin (referenced as Othin) is told in depth. In this story, it is said that “A sacrifice was to be made for a good season at the beginning of winter….” This first sacrifice was to Thor, and there were two more to Freyr and Odin. It then states that after Skadi’s failed marriage with Njorth, she married Odin and had many sons. In a verse about one of their sons, Skadi is directly referred to as “Ski-goddess.”
The Cliffs Notes of all this is that Ullr was cool but not cool enough to be remembered for what exactly it was that he did. On the other hand, Skadi was an influential giant who preferred the calls of the wolves to the sounds of the swans and possibly married the guy to whom sacrifices for a good winter were made.
It seems questionable what a good winter would mean, but after some further reading, it looks a snowy winter was somewhat important since they relied on the snow and ice for migration and travel. However, a fruitful summer seemed to be the main reason for sacrifices.
Modern times have brought on some alternative interpretations, of course. Over 100 years, skiers decided that Skadi and Ullr got married and that Ullr was the god of snow and worth making ski sacrifices to. Ski journals, websites, blogs, and even Wikipedia pages mention Ullr’s importance in the ski community and how we should praise him, hoping for a good season.
When you dig into Norse Mythology, there’s little doubt that Skadi was the goddess of snow and that she is the one you should be praising and sacrificing to for a strong winter, not Ullr.
Note: if you would like to sacrifice Thor properly, it seems you’ll need some live pigs or horses. You may want to consider alternatives.