In Russia we have very similar traditions - jumping over fire, sleeping with 12 field flowers and grasses under the pillow, the myth about the fern flower too. Also girls make flower wreaths and throw them in the river - they say that by how the wreath floats or sinks you can predict your love life. these aren't widely upheld traditions, but that's what's attributed to the day. Also it's traditionally the start of swimming season for most - in folk tradition it's because "all the rusalkas (mermaids) go hibernation" and won't drown you. But most likely it's due to the fact that in many regions the start of summer is rather cold and the rivers and lakes aren't warm enough for swimming. But since it's connected to St.Johns day, we celebrate it, as Christmas, 2 weeks later (July 7, Ivan Kupala day) because our church calendar is shifted. It's not an official holiday, but there are usually traditional art and craft fairs and other folk style festivities.
But since it's solstice today, I personally made some honey cakes, bought mead, and picked a tiny bouquet of field flowers to celebrate =) also working on an art for the comics, since it's about witches and all this stuff)) Actually plan to make a small story arc around the fern flower legend.