According to one website:
Ancient Pagans celebrated Midsummer with bonfires. “It was the night of fire festivals and of love magic, of love oracles and divination. It had to do with lovers and predictions, when pairs of lovers would jump through the luck-bringing flames…” It was believed that the crops would grow as high as the couples were able to jump. Through the fire’s power, “…maidens would find out about their future husband, and spirits and demons were banished.” Another function of bonfires was to generate sympathetic magic: giving a boost to the sun’s energy so that it would remain potent throughout the rest of the growing season and guarantee a plentiful harvest.
Today, Estonians celebrate Midsummer Eve and Midsummer (Jaanik) in a similar manner to ancient Estonians. They travel to the country, light bonfires, stay up all night drinking, singing, dancing, and jumping over the bonfires. Hey, if it ain’t broke…
Estonia was one of the last of the northern countries to be converted to Christianity, and even then many people continued to practice pagan rituals. Today, Estonia has no official religion, with about 60% of the population claiming a religious affiliation. Mixed in with the Midsummer holiday is St. John’s Day (Jaanipäev). This is the Christian holiday celebrating the birth of St. John the Baptist. This is a minor Christian holiday, and the name was basically just applied to a pagan holiday to “Christianize” it.
(thanks to illumineerija Rachel for this contribution)