Astronomical Explanation of the Summer Solstice:
The summer solstice is a somewhat inaccurate name since the Northern summer solstice is the Southern Winter solstice; however, because the larger part of English speaking peoples live in the North (and they're the ones reading this) I shall hereafter refer to the Northern summer solstice.
The summer solstice occurs when the Earth has finished tilting its Northern face towards the sun as a result of an annual tilt of the Earth's axis. After this, the Earth begins to tilt the opposite direction, but for this brief moment in time the sun is centered over the Tropic of Cancer approximately 26 degrees above the equator.
This long remove from the equator causes the summer solstice to be the longest day of the year, the reason for which is similar to the physics concerning the North Star. Nearly all stars, if you watch them long enough, set in much the way the sun does. They rise at a certain time of night (or day, I know, its crazy but we're not the center of the universe and things are still happening out there) and then fall below the horizon a period of time later. The North Star does not do this because it is set almost perfectly above the North Pole, which means that even though the earth rotates the Northern Hemisphere is always angled in such a way that everyone can still see the star. This effect causes the sun to stay around a bit longer too, but to a much lesser degree.
This event, the summer solstice, occurs around the 10th or 11th of June each year, which is encompassed within a period known as midsummer, though the actual Midsummer's Day occurs on June 24th. In fact, this process is so predictable that if you sat in a certain spot and made a pile of rocks directly between you and the setting sun on the day of the setting sun, you could see the sun in the same spot next year provided you and the rocks are in the same spot as before. Who knew!?! - besides those people who made Stonehenge, but they weren't particularly important.