Stockholm sits at roughly 59.3° north — further north than any point on the British mainland, on a line closer to southern Alaska than to anywhere most visitors have been. That latitude, not the temperature, is what makes planning here different, and it is why a summer visit and a winter one to the same city are barely the same experience.
The numbers
At midsummer, the sun is above the horizon for a little over eighteen and a half hours, and at its lowest point around midnight it is only about seven degrees below the horizon — not enough for the sky to go properly dark. Stockholm is too far south for true midnight sun, which needs the Arctic Circle, but it gets something almost as strange: a long, luminous, blue-grey night that never quite arrives.
At midwinter the arithmetic runs backwards. The sun is up for a little over six hours, and at midday it reaches only about seven degrees above the horizon — a low, raking light that never gets overhead, so mid-afternoon feels like evening.
The identical seven-degree figure at both solstices is not a coincidence. It falls straight out of the latitude and the tilt of the Earth's axis, and it is a neat way to remember how extreme the swing is.
Work out any date
The civil twilight figure is the one that actually matters for planning. It is how long the sun is either up or less than six degrees below the horizon — roughly the period when you can see properly outside without artificial light. In late spring and summer it is dramatically longer than the sunlight figure, which is why Stockholm evenings feel endless well before and after midsummer, and why an evening race here in June is run in full daylight.
Around the equinoxes the day length changes by roughly five to six minutes every twenty-four hours. Over a week's stay in March or September, that is a noticeably different evening on the day you leave from the day you arrived.
The calculator takes any latitude, so it also works for comparing Stockholm against a city further south. The difference is usually larger than people expect.
Season by season, honestly
Winter is dark, and the dark is the point rather than a drawback if you plan around it. Days are short, and afternoons end early enough that a full outdoor itinerary has to happen in the middle of the day. In exchange you get low golden light for most of the hours the sun is up, indoor culture at its best, and the possibility of snow and ice — possibility, not guarantee, because Stockholm winters swing between properly frozen and merely grey. Mid-December brings Lucia, when choirs and candle processions appear in churches and public buildings around the 13th.
Spring arrives late and then all at once. March and early April can still be bleak, and then the city turns over in about ten days: the light comes back fast, outdoor seating appears at temperatures other Europeans consider mad, and people occupy every south-facing surface.
Summer is the obvious answer and a very good one, with two things to plan around. Midsummer — celebrated on the Friday falling between 19 and 25 June — empties the city, as Swedes leave for the countryside and the archipelago and a meaningful share of shops and services close for it. And July is the national holiday month: the city is far from shut, but some things you had planned around will be closed for the summer.
Autumn is the underrated season. Late August and September have usable daylight, warm-enough water for anyone who swims at all, forests full of berries and mushrooms, and few visitors. It is the best time of year for the outdoor half of this site — see training outdoors here — and the light falls fast enough that you notice it change during a stay.
Packing for a latitude rather than a temperature
Two things follow, and neither is about coats. In winter, bring what you need to be outside in the dark: reflective gear if you are running, and a head torch if you plan to be in an unlit reserve, which will be most of the day. In summer, bring something to make a bedroom dark — hotel blackout curtains vary in quality, and 03:00 in June is bright enough to wake you if the curtain does not fit. If you are competing, that second one is not a comfort issue. It is a sleep issue, and sleep is the part of your preparation most likely to be destroyed by travel.