Night Of Ivan Kupala. Belarus, Russia, Ukraine

The Ukrainian, Belarusian and Russian name of this holiday combines "Ivan" (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 Kupala predates Christianity, due to correlation to a pre-Christian Slavic god or goddess. Due to the popularity of the pagan celebration that with time it was simply accepted and reestablished as one of the native Christian traditions intertwined with local folklore. On rare occasions it is spelled as Ivanna Kupala (Joann) representing the tradition as feminine.
The holiday is still enthusiastically celebrated by the younger people of the Eastern Europe. The night preceding the holiday (Tvorila night) is considered the night for "good humour" mischiefs (which sometimes would raise concerns of law enforcement agencies). On Ivan Kupala day itself, children are engaged in water fights and perform pranks mostly involving pouring water over someone.