The “allspel” is literally translated as “all play”: all participants are invited on the stage or in the central place to play some tunes together. The tunes are taken from the common repertoire and can be downloaded here. All tunes are traditional and in public domain.
The allspel is usually led by somebody who knows the tune and makes sure that everybody starts in the same key and at the same time.
Please note that Angela (from Senja, Norway) has updated the list of most allspel tunes in 2019 and added recordings for those of us who prefer to listen to tunes instead of sightreading.