Vetrnætr (anglicized as Vetrnaetr; Old Norse for winter nights) was a specific time of year in Viking Age Scandinavia. According to Zoega's Concise Dictionary of Old Icelandic, vetr-nætr referred to "the three days which begin the winter season". The term is attested in the narrative of some of the Fornaldarsögur, mostly to express passage of time ("as autumn turned into winter").
The exact term "winter nights" is not mentioned in the Ynglinga saga by Snorri Sturluson where (in chapter 8) the three great sacrifices of the year are prescribed:
Þá skyldi blóta í móti vetri til árs, en at miðjum vetri blóta til gróðrar, hit þriðja at sumri, þat var sigrblót. |
There should be a sacrifice at the beginning of winter for a good year, and in the middle of winter for a good crop, the third in summer day, that was the sacrifice for victory. |
Specific sacrifices held at the beginning of winter during the Old Norse period were álfablót and dísablót.