Today is the day of the earliest sunset of the year In most of North America. Locally here, sunset is at 4:50pm. So from today on through summer, the sunset occurs at ever slightly later times and our afternoons get sunnier and sunnier.
Sunset Day is different from the Winter Solstice on December 21st, which is the shortest day of the year. This is because of the tilt of the earth's axis and eliptical orbit.
A plot of the position of the sun at a given geographic location at the same time each day traces out a warped figure-8 curve over the course of a year. This curve is called the analemma.
For more on analemmas, see the Wikipedia entry:
And of course there's
for all your analemma needs.