[If you don't get that title, yer too darn young.]
Tonight marks the summer solstice, the midpoint of summer (and what you'll hear many people mistakenly call "the first day of summer"). The exact moment of the solstice occurs at 05:45 on June 21, but that's June 20th in my part of the US (11:45 p.m. for me in Mountain time).
What does this mean, exactly? Well, say you were to go outside every day and map the path of the Sun across the sky. It would make an arc, of course. If you were to note the height above the horizon where the Sun reaches the top of that arc, and continue to note it every day for a year, you'd see that at the summer solstice that apex is highest above the horizon and at the winter solstice it's at its lowest (southern hemispherites: swap those descriptions... and in reality it's a bit more complicated than this, I know, but this is close enough).
This change in the height of the Sun's path is because the Earth is tilted on its axis with respect to the plane of its orbit around the Sun. If you are so literal as to actually go out and measure the Sun's path on the sky, you'll note that starting today and going on for the next six months, the Sun will be lower and lower at local noon. When we reach the winter solstice on December 21st, the process will reverse, and off we go again.
So enjoy it now! For those of us north of the Equator, it means long days and short nights. As an astronomer that's something of a bummer, but then as a blogger I hardly ever go outside anyway. Except, of course, to buy eggs and stand them on end.