“Once in a Blue Moon.” We use this phrase when talking about something that happens on rare occasions. Today, astronomically speaking, the Blue Moon refers to the second Full Moon occurring in the same calendar month. Since there are about 29.5 days between Full Moons, every now and then the phases will line up so that the full phase will occur on the 1st or 2nd of a month and a second full phase will take place on the 30th or 31st.
Phil Harrington’s binocular universe: Tour Apollo sites through binoculars
June 2007: To celebrate the Blue Moon this month, let's tour our neighbor through binoculars.