"Earth Day is an annual day on which events are held worldwide to increase awareness and appreciation of the Earth's natural environment" (Wikipedia)
The female in the final frame is emptying the contents of a pair of child's trainers onto her kitchen floor. The amount of sand and dirt that they contained has been exaggerated for comic effect. She is saying that she doesn't need Earth Day to increase her awareness and appreciation of the Earth's natural environment; she has a young child who brings that environment into her house.
"Earth" has two meanings in English. It can mean the planet, or it can mean soil. A sandbox is a child's playpen that is full of sand (and soil). She is implying that, for her husband, environmental awareness is a cause for celebration on one day a year. For her, it is an everyday nuisance.