This is when the sun is at its peak and the sun Goddess's power is at its prime before starting the decline in the death of winter. It marks a significant point in the life/ death / rebirth cycle.
It is also known as Litha or Midsummer and is the longest day of the year the time when the sun is at its maximum elevation. The Solstice has been of spiritual importance throughout mankind's history, for the ancient Celts it was marked by bonfires to add to the suns strength, for the Christians it is the feast of St John the Baptist and is near the date of the Chinese goddess of light (Li) festival.
For some pagans it marks the point when the sun Goddess at the height of her fertility and power joins in union with the Horned God of autumn / winter, it is this union that creates the harvest fruits and the seeds and berries which hold the promise of future life. It is a time to celebrate growth but in the knowledge that it is the nature of all things to age and die to be reborn.
Pagan celebrations of the Solstice vary greatly drawing on many different traditions. In England many hundreds of Pagans and non pagans gather at ancient sites the most famous being Stonehenge to see the sun rise. Many other Pagan's hold small rituals in places connected to nature from gardens to woodlands