The name of the holiday was originally Kupala; a pagan fertility rite later adapted into the Orthodox Christian calendar by connecting it with St. John's Day which is celebrated on 24 June.
Kupalo/Kupala is a Slavic deity representing joy, mid-summer, peace, magic, water, and herbs. Alongside his sister Kostroma, they represented fertility. Kupalo/Kupala is celebrated at a festival called Kupala Night with rituals of purification through water and fire. The name Kupala or Kupalo is etymologically related to the verb kupati, "to wet" or "to bathe." (mythus.fandom.com)
This weekend, the jubilee, 20th edition of the Kupala Night takes place at the Pomeranian Dukes' Castle in Szczecin. Kupała Night - Midsummer traditions is a popular open-air event, which has been organized by the Szczecin Castle for many years in cooperation with the Triglav association.
Triglav (Polish: "Drużyna Grodu Trzygłowa") is an association dedicated to exploring and popularizing the culture of the Slavs of the early Middle Ages. It accomplishes this through the so-called "historical reenactment", i.e. action aimed at recreating selected areas of old culture, both in the material, spiritual and social dimensions. (tryglaw.org)
The Kupala Night event includes such attractions as fights of warriors, demonstrations of old rituals, concerts and fire shows.
Source: Poland Daily 24, facebook.com, zamek.szczecin.pl