However, occasionally the moon comes full thirteen times in a year. This was considered a very unfortunate circumstance, especially by the monks who had charge of the calendar. It became necessary for them to make a calendar of thirteen months for that year, and it upset the regular arrangement of church festivals. For this reason thirteen came to be considered an unlucky number. Also, this extra moon had a way of coming in each of the seasons so that it could not be given a name appropriate to the time of year like the other moons. It was usually called the Blue Moon. There are seven Blue Moons in a cycle of nineteen years. This year (1937) has a Blue Moon in August the same as 1918. In 1934 and 1915 Blue Moons came in November. The next Blue Moon will occur in May 1940 as it did in 1921. There was a Blue Moon in February 1924. In olden times the almanac makers had much difficulty in calculating the occurrence of the Blue Moon and this uncertainty gave rise to the expression "Once in a Blue Moon."
A second-hand misunderstanding of this passage led to the widespread belief that the second full moon in a month is called a Blue Moon.