During the Green Holidays, the Slavs honored the deceased who didn't die their own death, for example, suicide or drowning. They believed that on these days the dead come to the world of the living, so people often went to the cemetery, bringing flowers or food. In Slavic mythology, mermaids (in ukrainian "rusalky") are considered to be girls who drowned in the river. After death, they turned into mermaids, so another name for the Green Holidays is Rusalii. The Slavs made a straw effigy and threw it into the river or burned it in the field.
Mermaids come out of the water before the Trinity, Makovskyi, Kostiantyn Yehorovych.