Since I am French-canadian, this is a very important Feast day as St. Jean le Baptiste is our patron saint. I have vivid memories of the bonfires lit up in the park next to our home during my childhood.
In the Church calendar, only three birthdays are celebrated: Our Lord, Our Lady and St. John the Baptist. All the other Saints are honored on the day of their death or birth in Heaven. Exceptions are Our Lady, who was without sin from her immaculate conception and St. John, who was cleansed from original sin in the womb of his mother, St. Elizabeth. The Council of Agde in 506 confirms this feast as one of the highest in the Church life. Three masses, like at Christmas, were celebrated on that day.
It is celebrated on the 24th and not the 25th because of the way romans calculated their calendars: Christmas is 8 days before the kalends of January, the birth of St. John the Baptist 8 days before the kalends of July and since June has 30 days, it gives us the 24th of June as the date for this feast. It is a feast of light, the longest day of the year (approximately). Bonfires are a tradition in many countries (including my native Quebec).
It also means that the days are going to become shorter until Christmas. "He must increase: but I must decrease" John 3:30.
St. John the Baptist is the saint of adulthood, of maturity. He shows us how to prepare the way of the Lord in repentance. His ministry is intimately linked to the sacrament of Baptism and to the sacrament of Reconciliation. Like him, we are also called as Christian parents to prepare the way of the Lord in our home (and sometimes it feels like preaching in the desert!), so that our children will recognize the Lord in their own life.
"...the Lord willed to announce to men His own coming through the Baptist, lest if He appeared suddenly, they would fail to recognize Him." - St. Augustine
For the domestic church, if you are not up to a bonfire in your backyard, decorating your door with lilies, St. John's wort and birch leaves is a nice way to mark the day of his birth. I also serve a barbecue (fire!) and corn on the cob that day, as my parents did.