The sources for this celebration are plentiful in the Old Norse sagas, to name a few:
Ynglinga Saga, c. 8:
“Óðinn setti lǫg í landi sínu, þau er gengit hǫfðu fyrr með Ásum. […] Þá skyldi blóta i móti vetri til árs, en at miðjum vetri blóta til gróðrar, it þriðja at sumri, þat var sigrblót.”
“Óðinn set laws in his land, that had formerly been upheld by the Aesir. […] Then they had to sacrifice in the beginning of winter for a good year, in the middle of winter for a good crop, and a third time in summer, that was a victory sacrifice.”
Óláfs saga helga (Saga of Saint Olaf), c. 107:
“Þat haust váru sǫgð Óláfi konungi þau tíðendi innan ór Þrándheimi, at bœndr hefði þar haft veizlur fjǫlmennar at vetrnóttum. Váru þar drykkjur miklar. Var konungi svá sagt, at þar væri minni ǫll signuð Ásum at fornum sið. Þat fylgði ok þeiri sǫgn, at þar vær í drepit naut ok hross ok roðnir stallar af blóði ok framit blót ok veittr sá formáli, at þat skyldi vera til árbótar. Þat fylgði því, at ǫllum mǫnnum þótti þat auðsýnt, at goðin hǫfðu reizk því, er Háleygir hǫfðu horfit til kristni.”
“That fall, the king received news from Inner Throndheim about farmers holding well-visited sacrificial feasts on the Winter Nights. There was much drinking. It was reported to the king that the cups were blessed honoring the Aesir according to the ancient custom. Furthermore, cattle and horses were slaughtered, and the altars sprayed with their blood. The sacrifices were accompanied by prayers for a bountiful harvest. It is also reported that all people knew that the gods were irate because people from Halogaland converted to Christianity.”
Gísla saga Súrssonar (The Saga of Gisli the Outlaw), c. 10:
“Það var þá margra manna siður að fagna vetri í þann tíma og hafa þá veislur og vetrnáttablót en Gísli lét af blótum síðan hann var í Vébjörgum í Danmörku en hann hélt þó sem áður veislum og allri stórmennsku. Og nú aflar hann til veislu mikillar þá er svo líður stundum sem áður var getið. Hann býður til veisluþeim báðum nöfnum, Þorkatli Eiríkssyni og Þorkatli auðga og mágum sínum, Bjartmarssonum og mörgum öðrum vinum og félögum.”
“In those days it was the custom of many men to celebrate the coming of winter by holding feasts and a winter-night sacrifice. Gísli no longer sacrificed after he left Viborg in Denmark, but he still held feasts and showed the same magnanimity as before. And now he arranges the great feasts at the same times. He invites to that gathering both men with the same name, Þorkatli the son of Eirík and Þorkatli the wealthy, his brother-in-law, the sons of Bjartmar and many other friends and comrades.”
Gísla saga Súrssonar (The Saga of Gisli the Outlaw), c. 15:
“Þorgrímur ætlaði að hafa haustboð að vetrnóttum og fagna vetri og blóta Frey og býður þangað Berki bróður sínum og Eyjólfi Þórðarsyni og mörgu öðru stórmenni.”
“Thorgrim decided to have a harvest-feast on the winter-nights, and to sacrifice to Freyr. He invited his brother Bork, and Eyjolf the son of Thord, and many other great men.”
We find a mention of this time of the year even in other sources such as Bede's De Temporum Ratione from England:
“Unde et mensem quo hyemalia tempora incipiebant Vuinterfylleth appellabant, composite nominee ab hyeme et plenilunio, quia videlicet a plenilunio ejusdem mensis hyems sortiretur initium.”
“Hence, they called the month in which the winter season began ‘Winterfylleth,’ a name made up from ‘winter’ and ‘full Moon,’ because winter began on the full Moon of that month.”
Ynglinga Saga, c. 8:
“Óðinn setti lǫg í landi sínu, þau er gengit hǫfðu fyrr með Ásum. […] Þá skyldi blóta i móti vetri til árs, en at miðjum vetri blóta til gróðrar, it þriðja at sumri, þat var sigrblót.”
“Óðinn set laws in his land, that had formerly been upheld by the Aesir. […] Then they had to sacrifice in the beginning of winter for a good year, in the middle of winter for a good crop, and a third time in summer, that was a victory sacrifice.”
Óláfs saga helga (Saga of Saint Olaf), c. 107:
“Þat haust váru sǫgð Óláfi konungi þau tíðendi innan ór Þrándheimi, at bœndr hefði þar haft veizlur fjǫlmennar at vetrnóttum. Váru þar drykkjur miklar. Var konungi svá sagt, at þar væri minni ǫll signuð Ásum at fornum sið. Þat fylgði ok þeiri sǫgn, at þar vær í drepit naut ok hross ok roðnir stallar af blóði ok framit blót ok veittr sá formáli, at þat skyldi vera til árbótar. Þat fylgði því, at ǫllum mǫnnum þótti þat auðsýnt, at goðin hǫfðu reizk því, er Háleygir hǫfðu horfit til kristni.”
“That fall, the king received news from Inner Throndheim about farmers holding well-visited sacrificial feasts on the Winter Nights. There was much drinking. It was reported to the king that the cups were blessed honoring the Aesir according to the ancient custom. Furthermore, cattle and horses were slaughtered, and the altars sprayed with their blood. The sacrifices were accompanied by prayers for a bountiful harvest. It is also reported that all people knew that the gods were irate because people from Halogaland converted to Christianity.”
Gísla saga Súrssonar (The Saga of Gisli the Outlaw), c. 10:
“Það var þá margra manna siður að fagna vetri í þann tíma og hafa þá veislur og vetrnáttablót en Gísli lét af blótum síðan hann var í Vébjörgum í Danmörku en hann hélt þó sem áður veislum og allri stórmennsku. Og nú aflar hann til veislu mikillar þá er svo líður stundum sem áður var getið. Hann býður til veisluþeim báðum nöfnum, Þorkatli Eiríkssyni og Þorkatli auðga og mágum sínum, Bjartmarssonum og mörgum öðrum vinum og félögum.”
“In those days it was the custom of many men to celebrate the coming of winter by holding feasts and a winter-night sacrifice. Gísli no longer sacrificed after he left Viborg in Denmark, but he still held feasts and showed the same magnanimity as before. And now he arranges the great feasts at the same times. He invites to that gathering both men with the same name, Þorkatli the son of Eirík and Þorkatli the wealthy, his brother-in-law, the sons of Bjartmar and many other friends and comrades.”
Gísla saga Súrssonar (The Saga of Gisli the Outlaw), c. 15:
“Þorgrímur ætlaði að hafa haustboð að vetrnóttum og fagna vetri og blóta Frey og býður þangað Berki bróður sínum og Eyjólfi Þórðarsyni og mörgu öðru stórmenni.”
“Thorgrim decided to have a harvest-feast on the winter-nights, and to sacrifice to Freyr. He invited his brother Bork, and Eyjolf the son of Thord, and many other great men.”
We find a mention of this time of the year even in other sources such as Bede's De Temporum Ratione from England:
“Unde et mensem quo hyemalia tempora incipiebant Vuinterfylleth appellabant, composite nominee ab hyeme et plenilunio, quia videlicet a plenilunio ejusdem mensis hyems sortiretur initium.”
“Hence, they called the month in which the winter season began ‘Winterfylleth,’ a name made up from ‘winter’ and ‘full Moon,’ because winter began on the full Moon of that month.”