I guess it's been a while since we've had a good Internet/Facebook hoax.  This one has actually been making the rounds for about eleven years.  The claim is that on August 27th (Wednesday) there will be "two moons" in the sky, the second actually being Mars.

Sorry space cowboys! There's no double moon this Wednesday, in fact there never has been and barring some cataclysmic event, there never will be.  The fact is that Mars will NEVER appear to be as big as the moon (more like a pinhead compared to the Moon's basketball).  The other big fact is, it isn't even a full moon for the regular moon this Wednesday.  The August full moon happened on the 10th and the September one will happen on the 9th.

If you'd like to know more about what's not going to happen, you can click here.