Bastille day is all about when the peasants of France revolted against the authorities by storming the Bastille, a prison for political prisoners on 7th July 1789. The peasants were able to enter and overtake the building not because it wasn't well enough guarded or strong enough (It had extremly thick walls and there were lots of soldiers there) but because the governer of the Bastille at the time was useless at making desicions and so non of the soldiers knew what to do. Because of this, when the peasants demanded to be let in, the guards opened the door and let them walk right in!
The Storming of the Bastille is seen as the beginning of the French Revolution and there is a big celebration in France every year on the 7th July.