The date for celebrating the moveable summer solstice became fixed on the Day of St John the Baptist, thus enabling the Catholic Church to associate many of the ancient summer solstice customs with his worship. The solstice fires became the fires of St. John, whom Jesus called “a bright and shining light”. The early Christians had a deliberate policy of transforming Pagan celebrations into church occasions. Some of the representations of John are rather strange for a Christian saint. He is often depicted with horns, furry legs and cloven hooves, like a satyr or woodwose or satyr. His shrines too are often of a rather rustic nature, ostensibly because John was fond of wandering in the wilderness. It is possible that John not only took over a Pagan Midsummer festival for his feast day, but also the attributes and shrines of an earlier green god. Other midsummer symbols accumulated around St John and he was made the patron of shepherds and beekeepers.
In the Middle Ages, Christian mythographers declared that St John was born at the summer solstice at the time of the weakening Sun, announcing his own power would wane with the birth of Christ at the winter solstice, the time of the strengthening Sun,  associating them with the oak and holly respectively, perhaps drawing on earlier myth and folklore. The evergreen holly persists through the winter death-time and so was identified with Christ, the white flower emblematic of his purity, the prickles his crown of thorns, and the red berries the drops of his shed blood: “…of all the trees that are in the woods, the holly bears the crown” in the words of the old carol. 
Illustration Paul Mason
 Phillipe Walter, Christianity, the Origins of a Pagan Religion, Inner Traditions, Rochester, 2003
 John Williamson, The Oak King, the Holly King and the Unicorn, Harper and Row, New York, 1986