The last definition for a blue moon is the one that matters for us this month. It occurs when there is a second full moon in one calendar month. The time between one full moon and the next is nearly the length of a full calendar month, approximately 29.5 days. So, sometimes, if a full moon happens in the first few days of a month, then a second full moon can happen at the end of the month. This type of blue moon comes around every two to three years.