There are simply too many dogs and cats in the world. There aren't enough loving homes to go around. This overpopulation problem wouldn't exist if everyone spayed and neutered their pets. This is the day to make sure your own pets are “fixed” and to urge other people to do so as well.
There are candlelight vigils planned for tonight, possibly near you. Also, you can probably find free spaying and neutering services at human societies, adopt-a-thons, microchip clinics, and other activities. Find out more here.