What does "bitter moon" mean?

The bitter moon, a bad moon in folklore, is generally paid more attention to in fortune telling or proverbs.

Modern means that the month of birth is not so good, and whether there is a scientific basis is different for different people.

Actually, it's just a statement. Whether there is a scientific basis is that different people have different opinions. But even in fortune-telling stories, it is just a matter of worry and hard work. There is nothing particularly bad about it.