Words and phrases evolve, their meanings change over time as does society and the way in which we relate to Nature. Calendars have changed quite a bit over the centuries, depending on who ruled society. For example, ancient Mayan calendars were based on a lunar cycle, following the 13 full moons in a year with each month being 30 days (13 x 30=364), and the left-over day being referred to as “a day out of time”. (Source: http://mayankin.com/13-moon-calendar/)
Since it’s easier to track the phases of the moon (people can visually see the moon but they cannot look directly at the sun), planting schedules were organized around a lunar calendar. Additionally, the lunar calendar respected the feminine aspects of nature (menstruation) and integrated this natural way to track the passage of time into their calendar. Eventually society changed from a matriarchal society to a patriarchal society, and all references to the previous ways of life were obliterated.
Once the Romans conquered the ancient world, they wanted to impress their stamp on all human endeavors, and therefore created their own calendar based on the Sun, using solar cycles. “The Roman republican calendar was a dating system that evolved in Rome prior to the Christian era. According to legend, Romulus, the founder of Rome, instituted the calendar in about 738 B.C.E. This dating system, however, was probably a product of evolution from the Greek lunar calendar, which in turn was derived from the Babylonian. The original Roman calendar appears to have consisted only of 10 months with 30.4 days each, in a year consisting of 304 days. The remaining 61 days were apparently ignored, resulting in a gap during the winter season.” (Source: http://www.webexhibits.org/calendars/calendar-roman.html#anchor-republican)
In the 1800’s the Maine Farmers’ Almanac defined a blue moon as “the third full moon in a season with four full moons instead of the typical three”, but by 1946 it had evolved once more, becoming “the second full moon in a calendar month.”
Currently, it also refers to “a rare phenomenon that occurs after volcanic eruptions, forest fires or dust storms when tiny dust particles enter the atmosphere and scatter red light while letting through the blue light.” (Source: http://www.history.com/news/ask-history/what-is-a-blue-moon)