Semik is an ancient Russian holiday celebrated in early June. It usually fell upon the Thursday of the Rusalka Week (better known as Trinity Week, because it the week of Whitsuntide and Trinity Sunday). Its name is derived from the Slavic word for "seven", because the Rusalka week was the seventh week after Easter.
The Rusalki were believed to be at their most dangerous during the Rusalka Week, and were supposed to have left their watery depths for the fields and forests. Peasant women sometimes hung offerings to appease them. A cross, a magic circle, incense, garlic, wormwood, a pin or poker and verbal charms were used to render the rusalki harmless. Swimming was strictly forbidden, lest mermaids would drag the swimmer down to the river floor.
On Semik, funeral services were held for those who had not received a proper burial. Peasants decorated the insides and outsides of their houses with birch branches, and they selected a birch tree to decorate with ribbons and beads. The birch was usually left in the forest, but was sometimes brought into the village. The birch was seen as a symbol of vegetative power and stood as a focal point for girls' singing and dancing. Vows of eternal friendship were made here. This was probably a spring or fertility festival, having much to do with bringing in the good and banishing the evil.