It's the solstice! (No, it's not the equinox, like a certain radio station is alleging... the equinoxes have equal day and night... i.e. spring and fall, boys and girls...)
In case you were kept in a box under your parents' bed for your entire childhood, today is the day when the night is at its longest in the northern hemisphere (and at its shortest in the southern hemisphere). For people on the equator, it's just another 12 hours of daylight, just like every other day in history, although at noon the sun is as far south as it ever goes there and it will start moving north tomorrow.
There, don't you feel educated?