Or in French: la Fête Nationale.
It commemorates the storming of the Bastille in 1789, which you may recall as the beginning of the French Revolution. After the Revolution, France became a republic. There are three ideals of the republic for all French citizens: liberty, fraternity, and equality.
There are celebrations today all over France, and in many countries of the world. I don’t know of any in my local area, though several of the French restaurants in the city are promoting Bastille Day specials.
More on Bastille Day at Wikipedia.