It's an annual summer event -- getting e-mails announcing that this August, Mars will be as big as the full moon!!!!!
But before you go booking campgrounds and setting up Mars and moon parties in the hopes of viewing this spectacular wonder, you should know that the e-mail is a hoax, although it sounds like an unintended hoax.
First, the e-mail says that on August 27th, Mars will be as close as it has been in thousands of years and NO ONE ALIVE WILL EVER SEE THIS AGAIN!!!! And then some say it'll be so huge, it'll be as if the sky has two moons. Some e-mails even come complete with a fancy Power Point presentation from someone who will never get those three hours of his life back.
Let's break down where this e-mail breaks down.
The first problem is the date. Whoever started e-mailing this around truncated off the year. Mars did make its closest pass to Earth in thousands of years --- on August 27, *2003*. But every year, people assume it's the next upcoming August and send the e-mail around again.
Second issue is the size. The original story when it was in the news back the stated that Mars would appear as big as the full moon "at modest 75-power magnification" -- i.e., if you're looking through a telescope.
But the way the e-mail got circulated, there was a line break after "full moon" and the "at modest 75-power magnification" got relegated to the next line where, apparently, the entire human race read right over it and missed it.
So put the two together, and you have a spectacular story that is completely misleading :)
You can read more about the e-mail at snopes.com