A blue moon is the second full moon within a calendar month.This cannot be helped, sorry, plus I actually found it rather fun that it is not just a saying but an actual event
"How often is a blue moon?"
"How often does a blue moon happen? Normally blue moons come only about every two or three years. In 2018 unusually, we had two blue moons in one year and only two months apart – and one was a lunar eclipse! The next time we will get two blue moons in a year will be 2037"
(since we must have one now, we'll see you in a year or two )