The Kupala Night (the midsummer fire-and-water festival)
The 'Kupala' night (the midsummer solstice, around June 23/24) is a well-attested folk fire-and-water festival across Slavic lands: bonfires are lit, people leap over the fire, the 'Kupala' flower (the fern-flower) is sought, and the water is blessed [S5]S5 · 19th-20th c. Slavic ethnographic collections (Volkov, Ivanov, etc.)19th-20th c.academic. The 'Kupala' is associated with Jarilo (the spring/youth/sun god), though the relationship is debated [S5]S5 · 19th-20th c. Slavic ethnographic collections (Volkov, Ivanov, etc.)19th-20th c.academic[S6]S6 · Modern comparative reconstruction (e.g., Ivanov & Toporov, 'Theses on Slavic Mythology')1970s-2000sacademic.
Pre-Christian Slavic religion left no native written corpus. Named gods are attested (Vladimir's pantheon, 980 CE; chronicles; ethnography), but the narrative layer is largely scholarly reconstruction (Ivanov–Toporov et al.). Every reconstructed claim is badged as such.
The 'Kupala' is a well-attested folk festival; its relationship to Jarilo (the sun/spring god) is debated [S5]S5 · 19th-20th c. Slavic ethnographic collections (Volkov, Ivanov, etc.)19th-20th c.academic[S6]S6 · Modern comparative reconstruction (e.g., Ivanov & Toporov, 'Theses on Slavic Mythology')1970s-2000sacademic.