Her widower Jim Sillars, Scottish Health Secretary Alex Neil and actress Elaine C Smith are expected to lead tributes at Edinburgh's Assembly Hall on April 25.
Ms MacDonald, 70, who suffered from Parkinson's disease, died on Friday in Edinburgh and will be cremated in a private family ceremony.
She represented the Scottish National Party at both Westminster and Holyrood, before latterly serving as an independent MSP.
Ms MacDonald, who represented the Lothian region, had been campaigning to legalise assisted suicide.
The memorial "is not to be an occasion for mourning", according to her family, who are preparing a playlist of her favourite country music songs and have urged those attending to wear bright clothes.