Midsummer is Scandinavia's most popular festival right along with Christmas. A traditional celebration of the Summer solstice, Midsummer is the longest day of the year (June 21). In Sweden, Midsummer is even celebrated as a national holiday (also see Scandinavian national holidays). Most Midsummer's Eve celebrations take place on the Saturday between June 20 and June 26.
The celebration of the Summer solstice is a very ancient practice, dating back to pre-Christian times. Midsummer was originally a fertility festival with many customs and rituals associated with nature and with the hope for a good harvest the coming fall/autumn.
In Sweden, where the festival is called "Midsommar", houses are decorated inside and out with wreaths and flower garlands. Swedes then dance around the decorated midsummer pole while listening to traditional folk songs known to all. In Sweden, as in many other countries, the magic of Midsummer includes bonfires (which reminds of Swedish Walpurgis Night traditions), and divining the future, especially one's future spouse!
In Norway and Denmark, Midsummer's Eve is also a popular day, celebrated with large bonfires and processions in the evening. In Norway, Midsummer's Eve is also called "Jonsok".
Among Danes, it's not just Midsummer's Eve but also Sankt Hans aften (St. John's Eve) which they celebrate on the eve of June 23rd. On that day, Danes sing their traditional "We Love Our Land" and burn straw witches on bonfires. This is done in Denmark in memory of the Church's witchburnings of the 16th and 17th century.
The Scandinavian Midsummer traditions stem from pagan times, showing the defeat of darkness to the powers of the sun god.
Like in every major Scandinavian tradition, celebrating with others goes hand in hand with good holiday food. Traditional food for Midsummer in Scandinavia are potatoes with herring or smoked fish, fresh fruit, and probably some schnapps and beer for the adults!
If you are interested in many more Scandinavian traditions, take a look at Annual Events & Traditions. You can also look up events and tips for the month of your travel choice in Scandinavia by Month, e.g. June - which is also the time of the Midnight Sun!