Rusalnyi Week comes before the midsummer Kupala festival and after Easter, in June, as spring turns to summer. Like Kupala, it is a magical period with pre-Christian roots.

The Green Holidays

In a church decorated with greenery, an Orthodox priest blesses people who have brought flowers on Trinity Sunday, also known as the Green Holidays (and elsewhere as Pentecost or Whit Sunday). Birch branches adorn icons on the feast day, and church floors are strewn with grass. Rusalnyi Week begins the following day.
video: Iryna Nevhad

Vladyslava Bondar, in 2020, cycling home with linden and maple branches to decorate her family's home in the Poltava region of central Ukraine.

Defensive decorations

Like the night of Kupala, the period around Trinity Sunday was believed to be a time in which the boundary between the natural and the supernatural worlds would soften, allowing spirits to roam across it. With summer approaching, it was the moment to ask the spirits for a good harvest. But some of those spirits, including souls of the dead, could be dangerous. To protect themselves, people would deck their houses with greenery, a custom known as klechannia. Maple, birch or linden branches were favoured for the outside; plantain, thyme and wormwood for the inside.

Ready for the rusalky

Rusalnyi Week marks the window in the year when rusalky, restless and troubled water spirits, were said to emerge onto dry land, venturing into fields and climbing trees. They were to be both feared and welcomed: they might tickle men to death, but they might also bring fertility to crops. To prepare for their coming, clothes and towels would be hung on the trees, so that they had something to wear when they came out of the water, and bread would be left in the fields so that they didn't consume the wheat. At the end of the week, a procession of villagers would symbolically escort them back to the water.

One more thing: we'd like to make it clear that despite what Google Translate and scholars who ought to do better may tell you, rusalky are not mermaids! They were turned into mermaids by 19th-century authors who imposed mermaid images onto rusalka folk traditions.
In those traditions, rusalky have entirely human forms, not half human and half fish.

Sure, they have a lot in common with mermaids – female, beautiful, dwelling in water, luring men to their deaths. But they have richer and more complex natures. The spirits of women and girls who met untimely or unnatural deaths, they are conflicted and tragic. Having died too soon, they don't know how to behave with humans – when they tickle people to death, they might just be trying to play with them.

For a rich and subtle discussion of rusalka nature, see this paper by Jiří Dynda.