The Climate -- Sweden's climate is hard to classify because temperatures, influenced by the Gulf Stream, vary considerably from the fields of Skåne to the Arctic Circle wilderness of Lapland.
The country as a whole has many sunny days in summer, but it's not super hot. July is the warmest month, with temperatures in both Stockholm and Gothenburg averaging around 64°F (18°C). February is the coldest month, when the temperature in Stockholm averages around 26°F (-3°C). Gothenburg is a few degrees warmer.
It's not always true that the farther north you go, the cooler it becomes. During summer, the northern parts of the country -- from Halsingland to northern Lapland -- may suddenly have the warmest weather and the bluest skies. Check the weather forecasts on television and in the newspapers. (Swedes claim these forecasts are 99% reliable.)
Summer -- When it comes to weather, the ideal time to visit Sweden is from June to August. At this time, all its cafes and most attractions, including open-air museums, are open, and thousands flock to the north of Sweden to enjoy the midnight sun. (However, except for special festivals and folkloric presentations, the major cultural venues in Sweden, including opera, dance, ballet, and theater, shut down in summer.) Summer also is the most expensive time to fly to Sweden, as this is peak season. To compensate, hotels sometimes grant summer discounts. (It pays to ask.)
Spring & Fall -- The months of spring and autumn, notably May through June and the month of September, are almost prettier than the Swedish summers. When spring comes to the Swedish countryside, wildflowers burst into bloom after a long dark winter.
Winter -- Scandinavia's off season is winter (about Nov 1-Mar 21). Many visitors, except those on business, prefer to avoid Sweden in winter. The cold weather sets in by October, and you'll need to keep bundled up heavily until long past April. However, other more adventurous tourists go to Sweden in spite of, or even because of, the winter. Students have returned to such university cities as Stockholm and Lund, and life seems more vibrant then. Cultural activities also abound. Skiers also go to Sweden in winter, but we don't recommend it. It is pitch dark in winter in the north of Sweden, and the slopes have to be artificially lit. You'd be better off soaking up the alpine sun in Germany, Switzerland, or Austria.
Of course, one of the most eerie and fascinating things you can experience in Sweden is to see the shimmering northern lights, and they can be viewed only in the winter.
The Midnight Sun -- In summer, the sun never fully sets in northern Sweden; even in the south, daylight can last until 11pm, and then the sun rises around 3am. The best vantage points and dates when you can see the thrilling spectacle of the midnight sun are as follows: Bjoürkliden, from May 26 to July 19; Abisko, from June 12 to July 4; Kiruna, from May 31 to July 14; and Gällivare, from June 2 to July 12. All these places can be reached by public transportation.
Remember that although the sun may be shining brightly at midnight, it's not as strong as at midday. Bring along a warm jacket or sweater.