It's a day to celebrate someone who was a "light shining in the darkness"
FROM THE ARCHIVES
The Church celebrates three birthdays throughout the year, those of Jesus, Mary and St. John the Baptist. This is because all three of these births were “unstained” by original sin. It is piously believed (though not dogmatically declared) that St. John was cleansed of original sin in his mother’s womb when he “leapt for joy” at the sound of Mary’s voice (cf. Luke 1:44).
St. John’s birthday is celebrated on June 24, three months after the Annunciation and six months prior to the birth of Christ. The feast coincides with the summer solstice, the time of the year when the days are the longest.
After the celebration of St. John’s birthday the days become progressively shorter up until the winter solstice around December 25 when the days start to get longer. This corresponds to the words of St. John the Baptist, “He must increase, but I must decrease.”
One of many traditional celebrations associated with St. John’s birthday is the blessing of a bonfire the night before. This is done to commemorate the words of Zechariah in the Benedictus:
“In the tender compassion of our God
the dawn from on high shall break upon us,
to shine on those who dwell in darkness and the shadow of death,
and to guide our feet into the way of peace” (Luke 1:78-79).
St. John is often seen as a light that shines in the darkness, pointing to the true light of the world, Jesus Christ.
It is a tradition in many parts of the world to host a bonfire party and to have a priest bless the fire. Sometimes people will burn old things to symbolize the transition of the end of the Old Law to make way for the New Law in Christ. It is also an appropriate time to burn old sacramentals that are no longer of use (like old scapulars), since burning is one of the approved ways to dispose of blessed items.
Here is the prayer of blessing of a bonfire to be said by a priest, though a layperson could recite the prayers in a priest’s absence to help those present understand the connection between the bonfire and St. John the Baptist:
P: Our help is in the name of the Lord.
All: Who made heaven and earth.
P: The Lord be with you.
All: And with your Spirit.
Let us pray.
Lord God, almighty Father, the light that never fails and the source of all light, sanctify + this new fire, and grant that after the darkness of this life we may come unsullied to you who are light eternal; through Christ our Lord.
The fire is sprinkled with holy water; after which the clergy and the people sing the following Hymn: Ut queant laxis.
P: There was a man sent from God.
All: Whose name was John.
Let us pray. God, who by reason of the birth of blessed John have made this day praiseworthy, give your people the grace of spiritual joy, and keep the hearts of your faithful fixed on the way that leads to everlasting salvation; through Christ our Lord. All: Amen.