The science is fairly straightforward: The Earth's axis is tilted by about 23.5 degrees from its orbital plane. In other words, our planet spins around its axis like a spinning top, but its north pole is always "off vertical" by 23.5 degrees.
As the Earth orbits the sun, the North Pole is pointing preferentially toward the sun during the summer in the northern hemisphere (now), but points away 6 months later on December 20 or 21. For the northern hemisphere, when the north pole is tilted toward the sun, it's the summer solstice; when pointing away, it's the winter solstice. For the southern hemisphere, the opposite is true (i.e. when it's the summer solstice in the northern hemisphere, it is the winter solstice in the southern hemisphere).
Also, the summer solstice is the day that the sun can be seen at the highest point in the sky. At the summer solstice, the sun will be directly above the Tropic of Cancer (northern hemisphere) or the Tropic of Capricorn (southern hemisphere). If you are located between the Tropics, the sun will pass overhead (i.e. the zenith) in the run-up to summer solstice. At these latitudes, the sun will be directly overhead at different times of the year depending where you are.
Summer solstice is also a reminder about how lucky we are to be living on a planet with a tilt.
If the Earth spun vertically -- with no tilt relative to its orbital plane around the sun -- the equator would always have the sun directly overhead and the poles would be in perpetual twilight. Every day would be a summer solstice for the equator and winter solstice for the poles. There would be no seasons, the world as we know it would be a very different place; a lifeless desert around the equator and frozen poles equally as hostile to the life. I suspect there would be some pretty nasty weather in between; hot and cold air forming a huge cell of violent circulating air dominating the hemispheres.
So, if one was inclined to celebrate the times of the year, summer solstice should be added to it. After all, everything we have comes directly or indirectly from the sun. I'm not saying to necessarily worship the sun anymore than the celebration in December worships the Sun coming back from dying; I'd be giving thanks for our "tilted world".