WWF who sponsor this annual event do it to raise consciousness for the environment. That's fine, it is good to be conscious. The problem of course is that these eco-groups like WWF, want concerted government involvement in all aspects of our lives because we - humans - are the scourge of Planet Earth. Our technology, our fecundity, and resulting billions, they say has raped and pillaged this planet and stolen habitat from organisms great and small.
One of the most important biological concepts taught in any basic environmental course is "carrying capacity". Usually it is used to describe how many organisms can be sustained within an ecosystem. The entire planet of course is a finite ecosystem and it has a carrying capacity. So more people, less of some other creatures in a natural system. Human technology can distort carrying capacity and make a liar of Thomas Malthus, and it has. Even so there are limits, and groups like WWF that support conservation of habitat and creatures are by implication advocating the downsizing of the human population. Because of the limits to growth, in many respects it is a zero-sum game. It's only a problem if you are among those going to be "downsized". How is downsizing accomplished? One way is to suppress technology, the thing that gives humans the "edge" on planet earth. I wonder, is that the unstated goal of the eco-groups? A return to simpler times, less technical, more natural, organic, back to the earth. The symbolism is certainly there, turn off the power en mass world-wide, light candles to dispel the darkness and be conscious of your carbon footprint. It has a folksy attraction, singing 'round the camp fire, who doesn't like that? It sounds good, but, be aware of the implied and unstated, that part is worrying.
By the way here is what I said last year.