July 14th is Bastille Day in France, a national holiday. Even Google gets in on the act (google.fr, that is).
The Bastille was a Paris prison, by 1789 more symbolic than significant. It had a sinister reputation as the place where people were held on the sole and arbitrary power of the king. More importantly in 1789, it was a storehouse of gunpowder and arms.
On July 14, Parisian crowds overwhelmed the prison and it was surrendered. The event is seen as a significant early step in the French Revolution.
Fête Nationale became a holiday in 1880.