Definition: A Blue Moon is the second Full Moon in a single calendar month.*
There was a Full Moon on 2nd July at 11:09 (GMT)
There is a Full Moon tomorrow at 18:05 (GMT)
So tomorrow's Full Moon is a Blue Moon.
Blue Moons are rare, hence the phrase "once in a Blue Moon" meaning 'not very often'.
Blue Moons are rare because Full Moons are 29½ days apart, which is only just shorter than a month of 30 or 31 days.
There cannot be a Blue Moon in February because it's too short.
There are more Blue Moons in October, August and July than in any other months.
Blue Moons happen roughly every 2½ years. That's about 41 times a century.
The last Blue Moon was on 30th November 2001, and the next two will be on 30th June 2007 and 31st December 2009.
Very occasionally there are two Blue Moons in a year. This happens where there is no Full Moon in February. This last happened in 1999 and 1961, and will next happen in 2018 and 2037.
Very very occasionally the Moon really does look blue. This is a different sort of Blue Moon. The blue colour is caused by dust or smoke thrown high in the Earth's atmosphere by, for example, major volcanic eruptions like Krakatoa, Mount St Helens or Mount Pinatubo.
A special message to readers in Australia, New Zealand and the Far East: you don't have a Blue Moon this month. Tomorrow's Full Moon falls just after your midnight, which therefore becomes the full Moon in August. So you lot have a Blue Moon on 30th August instead.
* Be aware that the generally-accepted definition of a Blue Moon (the second Full Moon in a month) isn't traditional at all and was actually invented by a journalist 60 years ago. The magazine in question has apologised for its error. Proper history here.
Bluemoon, you saw me standing alone, without a dream in my heart, without a love of my own. Blue moon, you knew just what I was there for, you heard me saying a prayer for, someone I really could care for. Bom, bom bom bom, bom bom, bom bom bom. Bom bom, bom bom bom, a dang, a dang dang dang. A ding a dong ding. Blue moon!