​Ever wondered what a Blue Moon was? Here is the answer:​ Once in blue moon! You all must have heard and also used the phrase_ "Once in a blue moon" , but many of us may not know what is a blue moon, or rather when is a blue moon. If a full moon occurs twice in a calendar month, the second one is known as the blue moon. A lunar cycle is 28.53 days long. So if a full moon occurs in the very beginning of a month, chances are that there will be another full moon towards the end of that month. But such a phenomenon is rare, hence the phrase "once in a blue moon". Just for academic interest, here is the full moon~blue moon calendar for the next decade >>> 02 July 31 July 2015 02 January 31 January 2018 02 March 31 March 2018 01 October 31 October 2020