On the almanac page for August 1937, the calendrical meaning for the term "blue moon" was given.
That explanation said that the moon "... usually comes full twelve times in a year, three times for each season."
Occasionally, however, there will come a year when there are 13 full moons during a year, not the usual 12. The almanac explanation continued:
"This was considered a very unfortunate circumstance, especially by the monks who had charge of the 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."
