Earth Hour is a global action against climate change.
It started in Australia in 2007 and it has grown to a global day of action. Last year 135 countries across every continent took part.
Organisers hope this year will be even more successful.
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The memorial service will be attended by Prince Harry and Meghan Markle.
David Browning admits manslaughter by diminished responsibility but denies murder.
Four people have died and the suspect is still at large, police said.