"This is the second year of our 4.20 party, a date 'sacred' to the worldwide cannabis culture," spokesperson Julian Stobbs said.
According to Wikipedia, 20 April 20 or "4.20" has become a day of marijuana celebration, particularly in North America.
"We tap into a complete cross-section of South Africa, just as the plant does. It is truly a rainbow crowd of old young, black white, straight, gay, smoker and non-smoker alike."
Stobbs said South Africa had a "huge, vibrant cannabis culture" with the plant being used for various purposes, including recreational and medical reasons.
"Easily half this country use dagga. Cannabis is cannabis, whatever you use it for," he said. The fundraising party would be held in Johannesburg's trendy Maboneng Precinct from 11:00 to 22:00 on Sunday.
Around 3 000 people attended the event last year, and Stobbs this year's event was also expecting to be well attended, although a R50 entry fee had been introduced.
The 4.20 event would feature a restaurant and cash bar, DJs and graffiti artist tag teams creating a "legalise it" mural.
There would also be a small market selling "all things cannabis", but not the drug itself.
Fundraiser
The event aimed to raise money for Stobbs' and Myrtle Clarke's, known as "the Dagga Couple", legal challenge to South Africa's dagga prohibition laws.
"The enforcement of dagga prohibition clogs our courts, creates unwarranted criminal records and costs SA taxpayers millions every year.
"The roots of prohibition lie in racism and it leads to organised crime, a fact supported by extensive international research," Stobbs said.
The couple were arrested in August 2010 and faced a charge of drug dealing because they were in possession of more than 115g of dagga.
Their case was subsequently struck from the court roll, pending the outcome of their challenge to the legality of the dagga-prohibition legislation in the North Gauteng High Court in Pretoria.
"We have served summons on seven government departments to answer to our charge of unlawful legislation."
The couple were seeking to call in eminent international experts on the matter at the next scheduled court appearance in March 2015.
"We will demonstrate to the court that it is possible to re-legalise dagga without causing any disruption to society whatsoever."
"This is the next great civil movement," he said.