It began in Sydney as a climate change awareness campaign to "show the world" and has now been adopted across the globe.
Pioneered in Australia, the inaugural Earth Hour in 2007 had about two million people and 2100 businesses switching off their lights around the country.
Ten years later, about 7000 cities and tens of millions of people worldwide will power down on Saturday night.
"We were thinking it would be a great symbolic action to show the world," World Wildlife Foundation spokeswoman Kerri Major said about the event's origin.
"It was the start of a simple campaign and we didn't think it would grow to such a phenomenal global footprint."
The event has been criticised by some as a tokenistic gesture but Ms Major said the campaign was about bringing climate change to the forefront of discussion.
This year's theme is the next generation.
The WWF conducted a survey of 500 primary school-age children and found six in 10 were worried about environmental issues.
"That was anything from losing animals to extinction and food and water running out," Ms Major said.
"It was ranked higher than personal safety and having a career.
"The onus is on us to think of the legacy we're leaving."
One in four Australian are set to take part in this year's Earth Hour, and lights will be switched off for an hour at 8.30pm.
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