Members of the public, politicians, family and friends will gather at the Assembly Hall on the Mound tomorrow to pay tribute to the independent Lothian MSP who died on April 4 aged 70.
The venue - where the Scottish Parliament met during its first five years - can hold over 1,100 people and is expected to be full.
Ms MacDonald's husband Jim Sillars, Scottish Health Secretary Alex Neil and actress Elaine C Smith are all expected to share their memories.
Country music songs favoured by Ms MacDonald will be played as guests arrive, and The Proclaimers will sing Sunshine On Leith to bring the event to a close.
Humanist celebrant Dr Bob Scott will introduce proceedings.
An announcement of the event said "all are invited", adding: "This is not to be an occasion for mourning. At Margo's request, please attend in bright clothes."
Ms MacDonald, who suffered from Parkinson's disease, was a former SNP politician but spent much of her Holyrood career as an independent member.
She is remembered for championing many causes, including calls for assisted suicide to be legalised.
Her death drew an outpouring of grief from across the political spectrum and beyond.
Mr Sillars said his wife's legacy would ''speak for itself'', adding: ''She supported and inspired generations of idealists and campaigners who, like her, wanted Scotland to take its place in the world.
''Her talent acted like a magnet and she gave her time so freely to so many for so long.''
First Minister Alex Salmond, leader of the SNP, hailed her as ''one of the great rallying figures of Scottish nationalism''.
Her private funeral was held on April 11 and was attended by her family and closest friends, while a motion of condolence was passed in Holyrood on Tuesday.