I don't think it's meant to literally suggest they just went for the summer. Rather "summer job" ("sommerjobb") is commonly used in Norway to mean a second job done while you have time off from your regular duties. The "summer" bit included because the term is most often used for students who have an extra long summer vacation and so has plenty of time to take a short term job.

Same in Sweden and Denmark.

At least in crop agriculture the work tends to be concentrated during planting and harvest so there is a big period in between with nothing to do for most of the people involved in planting and harvest. The labor for at least some of the Egyptian pyramids were idle farmers who had free-time available for hire, iirc.

Nothing to do outside of crop agriculture in Viking times gives time for:

- Building housing and barns

- Chopping firewood

- Shipbuilding

- Wood working

- Weaving fabric (linen) from the summer harvest

- Ropes

- Leatherworking

- Metalworking/blacksmithing

- Pottery

- Animal husbandry

- Hunting

- Fishing

... and probably lots of other things that were necessary during those times. Of course Viking raids are a different means of acquiring a lot of the above.

In Scandinavia there is zero agriculture work to do when the ground is frozen and covered in snow.

These guys were soldiers, right? When there wasn't fighting to be done in the North, they headed south to look for work.