Today (December 18) at 23:45 UT (6:45 pm. Eastern US time), Mars will make its closest approach to the Earth for the year.
As the two planets circle the Sun, they are sometimes close to each other, sometimes far (picture two cars speeding around a racetrack, with the inner car (Earth) overtaking the outer one). Right now, as Earth passes inside of Mars, the two planets are about 88 million km (55 million miles) apart.
This is called opposition, because Mars is directly opposite the Sun in the sky when this happens. Update: Oops! Opposition actually happens on the 24th; the shape of Mars' and Earth's orbits sometimes means opposition doesn't happen at closest approach. Thanks to Tom in the comments below for pointing this out to me. And while it's the closest Earth and Mars will be for a while, this is not a particularly close approach. The orbit of Mars is pretty elliptical, and right now we happen to be passing it when it's farther from the Sun than usual.
The image above was taken by Hubble yesterday, on December 17 (and let me congratulate the Hubble Heritage Team on the speed with which they released this image-- that's amazing!). They're releasing it in honor of the opposition event.
Now, let me be clear. Mars is small, and far away. Even through a decent telescope it's not much more than an butterscotch-colored disk, with only a few features detectable (usually a bright icecap, or the faint smudge of a large area like Syrtis Major, the triangular region on the right of Mars in the picture). I'm not saying you shouldn't take a look if you get the chance! It's worth it just to see an alien world with your own eyes. Just be prepared to not see what Hubble sees.
And remember: Mars will not look as big as the Moon! But it is still a burning red beacon in the east shortly after sunset. If you go out around 9:00 p.m. or so you'll see it rising in the east, with Orion and the Pleaides looming nearby too (on the right if you're in the northern hemisphere, or if you're standing on your head south of the Equator). It's a pretty scene, so even if you don't have access to a telescope, go outside and have a gander.