While several people have mentioned the connection between May Day and organised labour, I haven't noticed specific mention of anti-globalisation protests tomorrow.
also: http://www.mayweek.ab.ca/
(useful summary of May Day heritage)

This is my favourite time of year. The days are getting longer, the birds singing more. Despite the infernal call of the cuckoo, I really miss Skye at this time of year. I spent two years there before returning to city life. Still, there is enough greenery around me here to appreciate the fresh pale green leaves on trees, the flowering of cherry, blackthorn, horse chestnut, lilac, laburnum, azalea, & the first "rhodys" of the season. (are there any songs about hay fever?)

Somewhere in this thread - or in a similar one - someone in Australia pointed out that
seasons are opposite in the southern hemisphere. Even as a child I sometimes pondered whether people in the south had snow motifs on their Christmas cards, but now I am starting to think more about the topic. We have a lot of Christian celebrations superimposed on older festivals. The older pagan festivals were closely related to seasonal changes, contrasted with Christian commemorations of particular events or saint's days - which are then transported to other regions of the earth on the same date. Thus Christmas isn't celebrated on the southern winter solstice, but in December (or Jan. in some churches). I suppose for the smaller festivals, observation of the fests are more localised and people can all celebrate their own special saints. So I wouldn't expect May Day to be a big event in Argentina (but I could be wrong).

By the way, I was in Ghana Dec. 1989. Ghana is just north of the equator and the daylength doesn''t vary much. They have wet and dry seasons rather than hot and cold or short days and long. I noticed huge banana fronds outside the market stalls. When I asked, I was told these were Christas trees. I wonder are they used for palm Sunday as well.