Opposition Information
This year the Mars opposition will occur on 24 December. Mars will have a visual magnitude of -1.63 and will have a size of about 15.8 arcseconds. At opposition, it will be at coordinates (J2000) 06h11m, +26°45' in the constellation of Gemini. It will appear as a bright read-orange object that should rise at about the same time the sun sets (and set at about the same time as the next sunrise). The season on Mars will be early spring in the northern hemisphere and early autumn in the southern hemisphere; LS=7.1° for those familiar with that notation. At opposition, the entire visible disk of Mars is illuminated; it is in its full phase, much like a full moon. Before and after opposition, it will be in a gibbous phase; it can never have a crescent phase since Mars is further away from the Sun than Earth and thus can never be between Earth and the Sun.
Due to the relatively high ellipticity of Mars' orbit, the opposition date will not be the date of closest approch. The closest approach will occur on 19 December at which point Mars will be only about 55 million miles (about 88 million km) away. Since it is closer, it will be slightly larger than at opposition with a size of 15.9 arcseconds. One would expect that closer and larger would imply brighter, but since it is not a full phase, it only has a visual magnitude of -1.61 (remember, astronomers are a backwards sort so smaller, or more negative, magnitudes are brighter).
For some more detailed information on the opposition, you can check out a paper by Jeff Beish or the A.L.P.O. Mars Section web site.