The Dansebandfestivalen (dancing band festival = swing and Swedish swing music) is a big yearly evnet at Seljord. Major Scandinavian artists show up every year and there is much merriment and divorce (afterwards). Check the web site for details.
If you can stand country music with a fair Norwegian twist and much booze, the Seljord is your place. The biggest c&w music festival in the Nordic countries is held here yearly. This year's festival is just over, so better check the web site for the next year's events. If you're into it, it's cool, because the festival really attracts some major artists, like Bobby bare and Chely Wright this year.
Not a family event, this thing has 18 years age limit.
Due south of Seljord is the station town of Bø (Boe). Trains from Stavanger/Kristiansand (west) and Oslo (east) stop here. Nor many, but a few times a day. Mostly there is a bus service corresponding with the train arrivals and you jump on this, pay NOK 65 for the 30-km trip to Seljord. From Oslo to Bø it's about 1 hr 40 mins, and from Bø to Seljord it's another 35 minutes. The train option is by far the fastest, but not always the most convenient due to occasional hitches in the correspondence times at the station. Check updated tables before you go.