The night before June 24 was believed to have magical powers and people tried to harvest all the magic the St. John’s Eve (Svatojánská noc) could offer.
People in different parts of the world would attribute special powers to the night of the 23th of June. In what is now the Czech Republic, the church tried to eradicate these beliefs by substituting them with Christian celebrations (such was the case with Letnice and other traditions). As you maybe already know about the Czech traditions, many of them withstood the pressure and adapted.
Speaking of this special night, the Catholic Church assigned the whole day of June 24 to St. John the Baptist because at those times, the beginning of the day was considered the sundown of the previous day. And so, the St. John’s Eve got its name. However, this didn’t stop people from believing into the magic of the night and performing the once-a-year rituals even though mixed with Catholic beliefs (this was a common form of adaptation thanks to which many folkloric traditions survived).
Most common was collecting herbs at this night because they had special powers when cut at this very moment. Everyone who considered themselves a witch or a healer would collect the so called svatojánské koření (St. John’s Spices). Unmarried girls would make crowns from nine types of herbs, put them under their head at night, throw them in water the next morning and ask St. John to show them their husbands.
Some households prepared a bed for St. John which was a thyme-stuffed pillow placed under the table and a picture of St. John was placed on the pillow. They would tell their children that the saint came to rest at their home and left them small gifts.
A tradition that dates back to the pagan solstice traditions are the St. John’s Eve Fires (Svatojánské ohně). Although originally made for magic and cleansing purposes, the fires later became a fun event for youngsters. A young fir tree is adorned with flower crowns, firewood around its trunk and young people jump over the flames.
Here is a Czech documentary about the folkloric traditions of the St. John’s Eve.