Probably best to check with the organiser to see what sort of dress code they have in mind.
I have noticed on this site, that American style Burns suppers often have the men in very formal kilt outfits complete with Prince Charlie jackets, evening sporrans with cantles, and the ladies in tartan hostess skirts.
This is indeed authentic as some Burns suppers in Scotland do nowadays follow this format.
However most Burns suppers which I have attended here in Scotland have been much less formal. At the Burns supper which I attended last night there was a mix of modern smart casual and Burns era vintage style clothes. Bear in mind that Robert Burns was a lowland Scot and that the lowlanders did not wear kilts in the times when he lived. Last night I was the only gent wearing a kilt to my Burns supper, which I wore semi-casual, Argyle jacket shirt and necktie, but a plain leather day sporran and plain hose with no flashes. That said, a lady in a kilt or a tartan skirt, or a man in a kilt, should never go wrong at any Burns supper.