The date rule, what winter road conditions actually mean, and the penalty if you drive illegally.
In Sweden you must use winter tyres during a specific period, but only when winter road conditions actually apply. The date alone isn't enough.
Under Trafikförordningen 4 kap 18 a § a car must have winter tyres or equivalent equipment from 1 December to 31 March, but only when winter road conditions actually prevail. Outside that period the requirement does not apply at all, whatever the road looks like. Both halves together are the rule. The date on its own decides nothing.
Snow, ice, slush or frost on any part of the road. The wording comes from Transportstyrelsen's general advice to TSFS 2009:19 3 § rather than from a hard statutory definition, and it is the police at the roadside who judge whether conditions apply where you are. In practice: a bare, dry road is not winter road conditions, even in January.
Common question: 'Are winter tyres required Dec 1 – Mar 31?' The answer alone is NO, the requirement only applies during winter road conditions inside that window. Read the question carefully.
Driving on summer tyres in winter road conditions during the mandatory window results in a monetary fine. In serious cases the vehicle may be barred from further use until the tyres are changed.