Geographically you want to be in the northernmost third of the country. The further north the better. Auroras in southern Sweden are few and far between. You also need to be away from any big cities to avoid light pollution. The three major cities Stockholm, Gothenburg and Malmö are both too far south and too light polluted for good auroras. Cities like Luleå and Kiruna have lots of nights with visible auroras every year, to give you an idea of where you want to be to see auroras. Make sure you have something to do if you don't see auroras.
As mentioned in the other answer, the theoretical best time for auroras is not exactly mid-winter but slightly before and after, but this variation doesn't matter much compared to the probability of clear skies, which is usually better the colder the weather is. January and februrary probably have the best probability to get that cold weather with clear skies.
Go do something touristy in an area with possibility of auroras, and if you see auroras it's a bonus. Otherwise it will be a slow, expensive and boring wait for nothing. The Jukkasjärvi ice hotel, the Jokkmokk market, or Skiing in riksgränsen are popular winter attractions in northernmost Sweden. Also consider places in northern Norway like Narvik or Tromsø which have very spectacular scenery. Note that depending on your preferred activity, you may want to wait a little longer into spring. The sun doesn't go up at all for a couple of weeeks in mid winter, making some activities tricky. For example, for skiing at Riksgränsen you want to wait until late march at least.
- Go above 65 deg north if possible
- Go in mid winter like jan/feb, march or april if you don't want constant darkness.
- Make saure to have something else planned apart from chasing auroras