I found several versions of Old Norse calenders when I was researching Vikings, and I found them fascinating. I used the old names of the months to structure my book Daughter of Fire and Ice and referred to them again in Sigrun's Secret.
The Norse world divided the year into the twelve lunar months, similar to our calender today. The difference is that each runs approximately from the middle of each one of our months to the middle of the next. And the names are far more colourful and descriptive of the time of year than ours. They are based mainly around the agricultural year which would have ruled the lives of most of the Norse. This version is specifically Icelandic and is taken from The Sagas of the Icelanders:
Harpa-month (mid April - mid May) Meaning unknown
Stekktíð or Lamb-fold time (mid May - mid June)
(Also known in some versions of the Norse calender as Skerpla)
Sólmánaður or Sun-month (mid June-mid July)
Miðsumar or Midsummer (mid July to mid August)
Heyannir or Hay-time (mid August to mid September
Haustmánuður or Harvest month (mid September to mid October)
Gormánuður or Slaughtering month (mid October to mid November)
Ýlir Meaning unknown (mid November to mid December)
Hrútmánuður or Ram month (mid December to mid January)
This month contains the festival of Jól (or Yule) and is sometimes known by the colourful name Mörsugur or Fat sucking month.
Þorri or Thorri month, meaning unknown (mid January to mid February)
Góa or Goa month, thought to be named after a forgotten goddess (mid February to mid March)
Einmánuður or Last month of Winter (mid March to mid April)
I love that the year started with spring, rather than with the depths of winter. And I adore all the names which I hope I've rendered correctly. I found they helped me greatly in my task of visualising the lives of the people I was writing about.