Gällivare has two short seasons separated by long shoulder months of half-light. Summer runs from late June to August: the midnight sun shines for over a month, the mosquitoes are aggressive, the lakes are warm enough to swim in, and the Inlandsbanan trains run. This is when most foreign visitors come.
Winter here is the real attraction for many — proper deep cold (minus 20 is normal in January and February), snow on the ground from October to May, and the aurora visible on about three out of four clear nights. Days are short: by mid-December the sun does not rise at all and there is only a couple of hours of twilight. The ski season at Dundret runs from late October through early May, the longest in Sweden. Avoid late October and late April — the dark transitional months when nothing much is happening.