Tonight, at 05:30 GMT (12:30 a.m. Eastern time), it will be the winter solstice. At that moment, the Sun reaches its southernmost declination as astronomers measure position on the sky. What that means to normal people is that every day until June, the Sun's arcing path across the sky will get a little bit higher, days will get longer, and people in the southern hemisphere will complain that I'm not including them in this description.
OK, fine: if you're in one of those standing-on-your-head countries, flip the description.
Anyway, the solstice marks the shortest day of the year in the northern hemisphere, more or less (the Earth's elliptical orbit around the Sun messes with that a bit, changing the length of the day). From here on out, the way the geometry of the Earth's tilt combines with our orbit around the Sun, our nearest star be up longer every day, getting more time to warm the ground, and eventually heralding the start of summer.
I've described this so many times now, it's probably easier just to send you to the various posts I've written about it!
... and the opposite one:
So go enjoy the longest night of the year; take a look at Venus in the west after sunset, Jupiter high in the south all night, Mars rising around midnight, Saturn rising a couple of hours later, and then finally the Moon and Mercury poking over the horizon at twilight. Quite a night for planets!
And if you think about it, look down. There's another planet there, too.