Because of slight variations in the Moon’s orbital period, and the number of leap days that intervene over a 19-year time span, the Metonic Cycle can be accurate only to within a day.
The Metonic Cycle worked well early in the 20th Century—in the years 1925 and 1944. But thereafter, using the cycle, the date of full Moon shifted a day to November 1st (in 1963, 1982, and 2001). But then, in 2020, it returned to October 31st. Making it a rarer sight, indeed.
Any time the Moon is technically “full” on October 31st (as it will be this year), it would also have to be a Blue Moon because the lunar cycle is only 29.5 days long.
There is an alternate definition of a “Blue Moon”—when there are 4 full Moons in a single season, the third is considered a “Blue Moon.”