June 21 is the summer solstice, also known as the first day of summer in the northern hemisphere, not to mention the longest day of the year.
It all has to do with the constantly changing tilt of the earth along its north-south axis. On June 21 the northern hemisphere is angled more toward the sun than on any other day of the year (just as during the winter solstice on December 22 it's angled more away from the sun than on any other day of the year).
Seasonal benchmarks like the summer/winter solstices and the spring/fall equinoxes held deep, mystical significance for the ancients, who situated some of their grandest monuments such that they were geographically aligned with these celestial events (e.g., the Great Pyramids of Egypt and Stonehenge in England). Indeed, some neo-pagan groups today Wiccans and Druids, for example fancy themselves to be upholding ancient traditions by celebrating the occasions as holidays.
Which brings me to a peculiar seasonal superstition that still survives today (I say "still," though in truth we don't really know how old it is), namely the belief that on one or both equinoxes the only two times of the year when day and night are of equal length it's uniquely possible to balance raw eggs on end which, of course, it isn't.
Come to find out, judging from the sudden uptick in popularity of my article on egg balancing on June 21 and December 22 every year, a lot of folks apparently believe the same "special phenomenon" applies on the solstices.
But in a word, it doesn't. Because it doesn't really apply to the equinoxes, either. There's no scientific reason why eggs should be easier to balance on one day rather than another. In point of fact, if you're patient and dedicated, you can balance eggs on end 365 days a year.
Sorry if I've left anyone with egg on their face. Happy summer solstice!