Hades had forbade his children from raising their mothers' ghosts. But every Mother's Day, Nico would break that rule and raise Maria di Angelo and Marie Levesque, but Nico could only do so for an hour. Any longer and their father would find out.
After he did it for the first time, he apologized to Hazel for not being able to do it longer. But Hazel just hugged him and thanked him. Saying, "The amount of time does not matter. What matters is that you went against our father's rule just to be happy. And that's all I care about."
And from that day on, it became a tradition of the two to do that every Mother's Day.
