People are going to witness second full moon (or blue moon) in a month on July 31, 2004 this year.
According to modern folklore, a 'Blue Moon' is the second full moon in a calendar month. Usually months have only one full moon, but occasionally a second one sneaks in.
Full moons are separated by 29 days, while most calendar months are 30 or 31 days long; so it is possible to fit two full moons in a single month. This happens every two-and-a half-years, on average.
According to NASA report, July has already had one full moon on 2nd. The next, on July 31, is--by definition--a 'Blue Moon'.