This is the time when the summer solstice is celebrated. This is the time of the longest day and the shortest night. It will next happen on 21-June-2017.
Just as with every other important day of the wheel of the year, christians attempted to take over midsummer with their own celebrations.
Some satanists see this as a weak day with all of the light and the short night. Many Wiccans recognize it with a feast or even with a bonfire - sometimes burning some aromatic herbs in the fire. Feeling the power of the goddess is easy on this day.
To me, I enjoy the long day and the warmth of the sun. I am reminded of the enlightenment that comes to us from Lucifer and Lilith - the light of freedom, the torch they held high that we might follow.
It is a day of strength, and a great bonfire at night should be commensurate with that daytime power. How you elect to recognize and practice on this day is up to you.
So mote it be.
High Priest and Magus
The Liberated Of Lucifer