havamal93 mentioned “Galdrabókin” (the witchcraft book) to me, so I looked it up a little, and found it’s a 1500’s Icelandic book which was translated by a Swede and republished a few times.
In the 1600’s when a farmer wanted to know when it would be the next full moon, he held up his arm. He pointed his pointer-finger towards the sun and spread out his thumb in line with the moon. Then he measured how many thumbs were in the space between the sun and moon. If the distance from the sun to moon was, for example, seven thumbs, it had been seven days since the new moon. After this it was easy to count out how many days remained until the full moon, or to the month’s end and the next new moon. The only thing that’s needed is that both the sun and moon are visible in the sky at the same time, a phemenon that happens regularly in the Nordics (except for in winter).
Iceland still keeps the Old Norse calendar afloat because they have at least one holiday (“þorri”) that uses it. A quick search found this in English: https://www.time-meddler.co.uk/icelandic.html
I came across a nice phase of the moon calendar on an Icelandic site, so if you want to know the phases of the moon simply go to this link and look on the right-hand side to find the calendar.
Key: Sun = sunday, mið = wednesday, fös = friday, lau = saturday.
Here is another moon calendar (in Swedish), which uses real photos: https://www.kalender-365.se/manen/mankalender.html
Må = Monday, On = Wednesday, Fr = Friday, Sö = Sunday