That is actually not something that happens all that often—the full moon falling on a Friday the thirteenth—and the last time it happened was 13 October 2000.
The next time will only be on 13 August 2049, which is a very long wait.
What are the odds of this happening? Bear with me while we do some arithmetic:
There are 12.37 full moons in a year (on average) and taking leap years into account, 365.25 days in a year. Therefore, the odds of any one day being a full moon is 3.39%
There are seven days in a week (duh) so the odds of any one of those being a Friday is 14.29%
Every month has exactly one 13th in it, which means the odds of a day being the 13th is 3.29%
To calculate the odds of any one day being all three these things at once, we multiply the percentages with each other and the result is .0159%, and to get the odds of one year having all this at once you multiply it by 365.25.
The odds of a year having a full moon on a Friday the 13th is a mere 5.81%, which is why it only happens about every 20 years, on average.
