Summer solstice has already come (for blogging on the solstice, see Rebecca, and Rebecca again, and at ScienceBlogs, Chaotic Utopia). However, the fixed holiday with which it is most closely associated is today. It's the Nativity of John the Baptist, which, as it used to be the closest major solemnity to the solstice , was for a very long time the feast of Midsummer. In the medieval period the celebration of this day was often very wild; it was the big let-loose before everyone had to buckle down for the long, hard work of preparing for winter. Liturgically, it's a breath of Advent in Ordinary Time.
UPDATE: Amy Welborn has a selection from Augustine on the day