Witold Pruszkowski “Rusałki” (“Rusalkas”), 1877
In Slavic mythology, rusalka (sometimes also called boginka) is a female demon inhabiting waters, forests and fields.
Rusalki usually appear as beautiful, naked girls with loose hair* (rarely as old and ugly women). They are souls of girls who died before the marriage. Rusalki appear during the new moon (and also before the Kupala Night), entice young men and kill them by tickling or frenzied dance.
In Rus’ (remember - Rus’ and Russia are not synonyms), at time of Green week, people used to celebrate Rusalki’s week.
*In all Slavic countries girls used to plait their hair and cover it with wreaths - it was a sign of maidenhood. Married women and girls used to cover their hair with headscarves or something like wimples (many people who don’t know Slavic folklore think that only elderly Slavic women used to cover their hair, which isn’t true). Maybe Slavs described their female demons as beautiful girls with loose hair, because it made them even more “inappropriate”/”abnormal”/”evil”/”impure”?