I would vote for it and I like the idea of a spring/autumn date. Its not going to be perfect but that's the way the solar system works. People don't necessarily have to be nude all day, but just do something publically nude, like a nude flash mob, or bike ride even!!!
Instead of "World" Nude Day...why not one for each hemisphere during their respective summers? I for one, when outside, like to enjoy the sun AND the water. March, in Maryland at least, has already had some warm days to at least be outside, but my friends up in NY are not so lucky. So, if we go with the World Day, spring or fall would would be the most fair for everyone and we would just have to hope for the best weather on that day. Just like the Skinny Dip last year...Maryland and surrounding states got rain that day.
I agree with this suggestion of hemispheric days but would prefere a summer date as I definitely want to be comfortable when nude. Spring and fall can be rather iffy here in Canada.