A record number of countries - 134 - have already started officially observing Earth Hour Saturday evening.
Electric lights will be shut down between 8.30 and 9.30 as a reminder about climate change and the need to reduce our imprint.
The lighting on landmarks across the globe - such as the Eiffel Tower, Niagara Falls, and the Sydney Opera - will be shut down for the period.
Countless private households will do the same.
In Bulgaria, lights will be shut down in official buildings accross the country. Buildings of significance, such as the National Theater and Library, will be also left unilluminated.