The French observe Labour Day (May Day - 1st May) more stricly than they do Christmas. Pretty much the entire city closes down on this national holiday. There are marches throughout the working class regions on Paris by all the major Unions. Perhaps the best place to observe the marches is from the site of the Bastille. It's not a huge event in the city but it can be quite interesting for a visitor to Paris if you know where to go.