Midsummer traffic jams are annual phenomena in Finland. Main roads leading out of the city areas begin to swarm with cars on the Midsummer Eve when people pack their cars with grill food and mosquito repellents to head to the countryside. The Finnish Transport Infrastructure Agency even publishes Midsummer Traffic press releases to help people avoid the worst congestions. The Finns take their Midsummer seriously.
Midsummer Eve is a public holiday and thus a day off for the general population, as the governmental offices and most businesses are closed. ManyFinns have a summer cottage where they gather to spend the midsummer enjoying sauna, skinny dipping, barbeque and bonfires with friends or family, while towns and cities become desolate. This might come as a small shock for a tourist arriving to Helsinki for midsummer.
But not to worry: Many restaurants stay open, and Helsinki has many activities to offer, such as Seurasaari Midsummer Bonfires, Midsummer cruises and Midsummer saunas. For more info, see myhelsinki.fi