What actually causes it
The midnight sun is a straightforward consequence of Earth's axial tilt combined with high latitude. Above the Arctic Circle, the tilt means the sun stays above the horizon for a continuous stretch around the summer solstice, instead of setting and rising each day. It isn't unique to Finland — the same effect happens across northern Norway, Sweden, Russia, Canada and Alaska — but Finland's chunk of it falls entirely within Lapland, its northernmost region.
Where it happens, and where it doesn't
Only the northern part of Finland, roughly the area at or above the Arctic Circle, gets a literal midnight sun where the sun doesn't set at all. Utsjoki, Finland's northernmost municipality, has the longest run of continuous daylight, around 73 days each summer. Rovaniemi, Inari and Sodankylä also experience it, for somewhat shorter periods. Further south, including Helsinki, the sun does technically set around midsummer, but nights become so short and bright that it can feel similar — a lingering, never-quite-dark twilight rather than true darkness.
What it's actually like
Constant daylight takes some adjusting to. Blackout curtains are a standard household item in Finland for exactly this reason, and visitors are often surprised by how disorienting round-the-clock brightness feels for the first few nights. Midsummer itself, juhannus, is Finland's biggest summer holiday, traditionally spent at a lakeside cottage with a bonfire, and the extended daylight is part of what makes it feel so significant — for details on that cottage tradition, see our deep dive on Finland's lakes.
The winter flip side: kaamos
The same geography that produces the midnight sun in summer produces its opposite in winter: kaamos, the polar night, when the sun stays below the horizon for an extended stretch in the far north and daylight shrinks to a brief blue-grey twilight. Finns manage it with candles, warm lighting, and a general embrace of coziness at home, plus the fact that the same dark, clear winter skies are exactly what make Lapland one of the more reliable places in the world to see the Northern Lights.