We have no specific evidence to show that this holiday was observed by the ancient Celts. We do though, find a word in Old Celtic, Medro-saminos, that means '(of) Mid Summer'. Midsummer is also known as Litha and Alban Hefin (from Wales - point of summer). "Litha" means "stone" and is associated with calendars.
A solstice is an astronomical point and, due to the precession to the equinox, the date may vary by a few days depending on the year. The summer solstice is the midpoint of summer, when the sun is above the horizon for the longest period, has its northernmost rising and setting, and its highest elevation at noon. It is the apex of the sun's splendor, the longest day and the shortest night.
-Taken from "The Wheel of the Year at Muin Mound Grove, ADF" by Rev. Skip Ellison