Banks, schools and other government offices will be closed but most shops will be able to trade across South Australia on the public holiday to mourn the death of Queen Elizabeth II.
The September 22 holiday was announced by Prime Minister Anthony Albanese on Sunday and endorsed by state cabinet on Monday.
Shop trading hours will be similar to Anzac Day with most allowed to open from 12pm to 9pm with the government to authorise an exemption to trading laws.
Normal public holiday penalty rates will also apply.
Premier Peter Malinauskas says the government is working through arrangements for elective surgery in public hospitals to ensure as many operations as possible can proceed.
The premier said a national day of mourning was appropriate considering the magnitude of the Queen's death.
"Given the unavoidably late notice of this decision, there will be some disruption for some South Australians," he said.
"The state government is committed to ensuring to the extent it is possible, we minimise these disruptions to people's lives."
Mr Malinauskas said the holiday would be an opportunity for Australia to come together as a nation and pay respect to somebody who had made an unrivalled contribution to public life.