"Március egy tavaszi hónap." is correct too. "az" means "that" there and it's often used in places like this. It's like saying "March, that's a spring month." I don't know, it's like "az" is taking over the role of "is" but this is not a scientific observation so take it with a pinch of salt. Point is, you can leave it out, you don't have to worry about it.