It's Palm Sunday for most Christians, but for Ukrainians and other Slavic Christians, it is Flowery or Willow Sunday. We use pussy willows instead of palms, for a variety of reasons. (Of course, technically because we use a different calendar, Willow Sunday isn't until next Sunday, but that's a whole other thing entirely). :) It is traditional to strike one another on the shoulder with the willow branch and say "It is not I but the willow branch which strikes you to remind you that in seven days is the Great Day". The willow was a well loved plant, heralding the coming of spring, and an immensely useful one too (did you know it has ASA in its branches?)
To get more info, check out this website that goes into some detail of the use of the Pussy Willow in Slavic religious and everyday tradition. And here it gives you some hints on how to propagate your own pussy willows. We've got a great bush on the side of the house, pictured here. As in ancient times, it is a wonderfully uplifting thing to see this sign that spring has come! :)