Midsummer is the biggest party of the year in Scandinavian countries.  So big that Sweden is considering changing their national day from June 6 to Midsummer.  Midsummer is technically on the longest day of the year (between June 19 and 21).  In Sweden it starts the Friday after June 19th. 

It is celebrated with picnics, piles of strawberries (on cakes or otherwise), lots of time in nature, bonfires, maypole dancing, and symbols of the sun everywhere.  If you have crayfish to boil, this is the time to do it.  But more than anything, plenty of time with family and friends.