Father's Day is on Sunday, a day to honor their contributions to their children's lives.

But what about families with single parents?

Certainly, there's Mother's Day, but since single parents take on the responsibility of both parents, shouldn't they get twice the honor?

Greeting card companies are starting to come around to that argument. This year, among their offerings is a card that reads, "You've been both a mother and a father to me."

Inside, it reads, "I don't know how you did it... but I sure am glad you did. Happy Father's Day, Mom. I love you."

Hallmark doesn't make Mother's Day cards for fathers, but Greeting Card Universe does, including one that reads "Happy Mother's Day, Mr. Mom."

Father's Day is the fourth-largest sending occasion, according to CNN.

Of the 11 million families with children under age 18, and no spouse present, the majority are single mothers (8.5 million), the Census Bureau reports.