In his book Deutsche Mythologie (1835), Jacob Grimm noted that Russians used the word kupala to describe the bonfires they lit at the summer solstice, and recorded that some people explained the word as the name Kupulo, a harvest god.
Gods such as Koleda and Kupala were constructed from misinterpreted names of popular Slavic folk festivals; Koledo was the Slavic name for Christmas processions of carol singers, whilst Kupala comes from Ivan Kupala (literally: John the Baptist), whose festivity day is celebrated at the summer solstice in many Slavic countries.
Although the word kupala (or kupalo) is usually explained as “bather” (from kupat(i) ‘to bathe’), some scholars claim that it is not an epithet of John the Baptist, but a name of a pre-Christian Slavic deity, derived from some other root.
According to Vyacheslav Ivanov and Vladimir Toporov, the name Kupala is derived from the same Indo-European root as the name of Cupid, Roman god of love, which means ‘passion’ or ‘desire’. The cult of Kupala, the god of fertility and sexuality, was presumably replaced by worship of John the Baptist.
According to some texts on Slavic mythology, Kapalo or Kupalo is the god of the summer solstice. Kupalo is the mature, the aging Yarilo. Yarilo comes into human world (Yav) every spring to bring new life, fertility and rich harvest. In the summer he turns into Kupalo. His life on the world gradually moves to its end.
He has accomplished his mission in our world and sets off for the Underworld, so he can return again next summer.
This is why the Kupalo festival (summer solstice) is actually bidding farewell to the old-aged Yarilo – a preparation for his later ritual burial. During the celebrations, for the last time people express their joy of god Yarilo’s visit in their world, the happiness he had brought; they sing incantations and prayers to the fertile god to come again next year.
The year is half-way through, last fruitful months are elapsing and then winter will come – the time of death goddess Mora, time of darkness, cold, misery, illness and death.