Monday is the Orthodox holiday of Whit.
Whit Monday or Pentecost Monday (also known as Monday of the Holy Spirit) is the holiday celebrated the day after Pentecost, a moveable feast in the Christian calendar. It is moveable because it is determined by the date of Easter. In the Catholic Church, it is the Memorial of the Blessed Virgin Mary, Mother of the Church.
Whit Monday gets its English name from “Whitsunday”, an English name for Pentecost, one of the three baptismal seasons. The origin of the name “Whit Sunday” is generally attributed to the white garments formerly worn by those newly baptized on this feast.
It is expressed in many different ways in Ukraine. In Western Ukraine, it’s called “Zeleni Sviata” or the green holiday, and there’s a tradition of putting cattail reeds in the window: