It meant then exactly what it means today, a fairly infrequent event, not quite regular enough to pinpoint. That's meaning number four, and today it is still the main one. The poor crooner's Moon often turns to gold when he gets his love at the end of the song. That's number six. It's not rare to see two full Moons in a month.
Months have different lengths, so the phenomenon moves around a bit.
Finally we arrive at the most recent meaning of all, the second full Moon in a month. I first heard it in 1988. Across North America the blue Moon caught the public's imagination. And that month we got calls about blue Moons. I searched high and low for an earlier example of this usage, or any other name for two full Moons in a single calendar month.