‘Blue moon’ occurs once in a blue moon. The event takes place when a second full moon happens within a calendar month. A moon cycle approximately takes one month, therefore it is strange that two full moons cycle through a predetermined period of one month.

This year such a blue moon will appear on July 31, for the reason that the last full moon occurred on July 2. It has been reported by Universe Today that the last time a blue moon appearing in any month was on August 31, 2012. As per reports, the next time blue moon will occur in January 2018.

The last blue moon that occurred in July appeared in 2004. It has been reported that the next July blue moon will not take place until 2034. As a result of the timing of the full moons, a blue moon appears nearly once every two to three years.

Some people think that the second full moon in a month is called “blue” for the reason that the Farmer’s Almanac labeled it in blue ink in place of black.

Sometimes, the moon appears blue, when it is observed from Earth. However, this phenomenon occurs as a result of light-scattering particles in the air, and is not associated with whether or not the moon is full during that time.

As per reports, moon-watchers could be more fascinated by the next total lunar eclipse, which will occur on September 28. The event that occurs "once in a blue moon" is here to create excitement among them.