On Ivan Kupala day, children are engaged in water fights and perform pranks mostly involving pouring water over someone. Many of the rites related to this holiday are connected with the role of water in fertility and ritual purification. Youths would jump over the flames of bonfires. Girls would float wreaths of flowers often lit with candles on rivers and would attempt to gain foresight into their relationship fortunes from the flow patterns of the flowers on the river. Men may attempt to capture the wreaths, in the hope of capturing the interest of the woman who floated the wreath.
There is an ancient Kupula belief, that the eve of Ivan Kupala is the only time of the year when ferns bloom. Prosperity, luck, discernment and power would befall on whoever finds a fern flower. Therefore, on that night, village folks would roam through the forests in search of magical herbs and especially the elusive fern flower Chervona Ruta.
Traditionally, unmarried women, signified by their garlands on their hair, would be the first to enter the forests. They are followed by young men. Therefore, consequent to the quest in finding herbs and the fern flower may be the blooming of relationships between pairs of men and women within the forest.
It is to be noted that biologists have held the persistent scientific fact that ferns have never and will never bloom.
The Russian name of this holiday combines "Ivan", Russian for the name John (the Baptist), and Kupala which is related to a word derived from the Slavic word for bathing, which is cognate. The latter is reinterpreted as John's baptizing people through full immersion in water (therefore his biblical title of the Baptist). However, the tradition of Kupula predates Christianity
Images from fiski.net and google images
Info from wikipedia