"Once in a Blue Moon" is a phrase used colloquially to mean "a rare event". Also, "Once in a Blue Moon" also associated with the rare occurrence related to the physical moon. However, as long as the event is rare, it is "Once in a Blue Moon". The once in a blue moon events associated with the physical moon is discussed in the following.
Once in a Blue Moon on Merdeka Day
Because the lunar cycle is 29.5 days, most months only see one full moon. But every once in a while a full moon, exactly occurs once every 2.7 years, rises twice in the same month and becomes a blue moon. For example, this year, the second full moon in a month occurred on Aug 31 after the first one on Aug 2. This year's Malaysia Merdeka Day as reported by Bernama falls on a blue moon. The next time we humans will see a blue moon is on July 31, 2015.
The Farmers' Almanac
The Farmers' Almanac defined blue moon as an extra full moon that occurred in a season; one season was normally three full moons. If a season had four full moons, then the third full moon was named a blue moon. Farmers' Almanac is an annual North American periodical that has been in continuous publication since 1818
For example, the first moon of summer is called the early summer moon, the second is called the midsummer moon, and the last is called the late summer moon. When a season has four moons the third is called the blue moon so that the last can continue to be called the late moon.
Visibly Blue Moon
The most literal meaning of blue moon is when the moon appears to a casual observer to be unusually bluish and the moon not necessarily a full moon. Of course it is a rare event to be able to visually see a visibly blue moon. The effect can be caused by smoke or dust particles in the atmosphere, as has happened after forest fires in Sweden and Canada in 1950 and 1951 and after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years.