Hundreds of millions of homes and businesses around the world will go dark Saturday night as part of Earth Hour, an annual event meant to raise awareness about climate change and the environment.

Now in its ninth year, Earth Hour encourages individuals and organizations around the world to turn off all of their non-essential lights for one hour. This year, it’s scheduled to start at 8:30 p.m. local time on Saturday, March 28.

Organizers say Earth Hour has become the world’s largest grassroots movement in support of the environment, and it has continued to grow with each passing year. More than 7,000 cities and towns in 162 countries and territories took part in Earth Hour in 2014. This year, the group behind the campaign says 172 countries are expected to take part. 

Whether you’ve participated in Earth Hour before or are thinking about taking part for the first time, here are a few things to know about it.

Creating awareness about climate change

The idea for Earth Hour was first conceived by the World Wildlife Fund in Australia in 2005 in response to scientific data revealing the devastating impact of climate change. The environmental advocacy group teamed up with advertising agency Leo Burnett and came up with the idea of a large-scale 'lights-out' event. Back then, the project was called "The Big Flick."

It was born in Sydney, but soon went international


A woman holds up a candle during an Earth Hour vigil in Mumbai, India, in 2011. Earth Hour, a global call to turn off all non-essential lights for 60 minutes in a bid to highlight global climate change, takes place worldwide this year at 8.30 p.m. local time. (Rajanish Kakade/Associated Press)

The inaugural Earth Hour was held in Sydney, Australia, in 2007 at 7:30 p.m. About 2.2. million people and 2,100 businesses across the city participated. It was deemed such a success that there were plans to make Earth Hour a national event in Australia.

Those plans were quickly expanded once organizers realized how much international interest the campaign had drummed up. Earth Hour went global the following year with 371 cities and towns in more than 35 countries taking part.

On March 5, 2015, Earth Hour received four awards at Marketing magazine's PR Awards, including "Best Digital Communication Strategy."

Held in March to coincide with the equinoxes

Earth Hour is held every year in late March, around the time of the Spring and Autumn equinoxes in the northern and southern hemispheres. This is when sunset times are almost the same in both hemispheres. Organizers say this ensures the greatest visual impact for a global 'lights out' event.

Major landmarks also ‘go dark’ for Earth Hour

Major landmarks around the world have taken part in Earth Hour, including the Sydney Opera House, the Great Pyramids, the Empire State Building and the Las Vegas Strip. Even the so-called City of Lights has gone dark, with the Eiffel Tower shutting off its power.

Lights on Toronto's CN Tower were turned off during Earth Hour's first international campaign in 2008, as thousands of people across the city joined in.  

IN 2015, about 40 UNESCO World Heritage Sites, including the Acropolis in Athens and Edinburgh Castle in Scotland, plan to go dark in support of Earth Hour.

Not to be left out, websites have also found ways to take part in Earth Hour. The Google homepage in many countries has 'gone dark' by switching its usually-white background to black.

Earth Hour Blue, a crowdsourcing and crowdfunding online platform that supports environmental projects, was launched in 2014 and has raised more than $61,000 US.

On Twitter, Earth Hour organizers want people to use  #yourpower as a hashtag.