I used that philosophy when my daughter was young. I would just tell her that it was a bad day and mommy was feeling down. I never wanted her to think it was her fault. She got to where when she would see me cry, "is it a bad day". All I had to say was yes and she would go play quietly and not take it personally. She is a school counselor now, and doesn't hold on to guilt like I did when I was a child, for which I'm grateful. She's still there for me when I'm depressed.