Sarek National Park is located in Laponia, Northern Sweden. Here you can experience probably the last remaining mountain wilderness area in Europe. Largely untouched, there is no infrastructure within Sarek: No huts, no signs, no managed trails, and only a couple of bridges to facilitate river crossing at some selected places. It is the ideal place to escape from civilisation and focus on the essential aspects of life.
If you are lucky, you may spot elks or even bears in the scrub of densely vegetated valleys. In any case, reindeers populate the more elevated areas during summer season. In the surrounding area of Sarek, the Sami people still follow their traditions and you should not miss the smoked fish and home-baked bread they offer.
You should bring all you need to survive in the wilderness, at least 10 days of time and also some trekking experience to be ready for the trip! Especially the river crossings can turn out to be demanding or even impossible, and having a plan b is essential. The so-called "bushwhacking" in head-high shrubs can be very strenuous and decreases your walking speed to less than 2 km per hour. Note: Sarek is definitely not the place you want to be during bad weather conditions! In case of weather change, it is reasonable to head to the next exit.
The best time to visit Sarek is to my mind the end of August. Then, mosquitoes are usually not that frequent anymore but night temperatures drop below zero only occasionally. However, the hiking season lasts from July to September. In July, old snow can still cause problems in some years, like in 2015, but the extremely long days are a fascinating experience. In September, autumn colours are wonderful, but you have to prepare for very cold nights already. The huts in the surrounding Padjelanta National Park or along Kungsleden are only run during the season.
For orientation, Fjällkartan BD 10 is essential. The online version can be studied here. Claes Grundstens "Pa fjälltur - Sarek" (original in Swedish) or "Sarek - Trekking in Schweden" (German translation) is the ideal guidebook describing possible route options.
Possible entrances to Sarek are Kvikkjokk in the South and Ritsem, Suorva and Saltoluokta in the North. Other possibilities are Staloluokta (helicopter from Ritsem or Kvikkjokk), Tarraluoppal (helicopter from Kvikkjokk) or Aktse (by car and walk/boat or helicopter from Kvikkjokk).
Entering Sarek via Kvikkjokk, it is probably the best option to take the night train from Stockholm. The other entrances can also be reached by a reasonable flight connection from Stockholm to Gällivare offered by NextJet. There is only transport by taxi from Gällivare airport to the bus terminal. It is very important to check the connection to local buses and boat transport beforehand, since they usually run only once or twice a day and only during summer season.