The lorry attack in the French city of Nice, which has killed at least 84 people, took place on Bastille Day.
The driver targeted large crowds who had been watching a firework display to mark the national holiday.
It signifies the start of the French Revolution, more than 200 years ago. It's France's independence day and it is celebrated every year on 14 July.
The Bastille was a prison where political dissidents were locked up.
In 1789 people were unhappy with King Louis XVI's reign, which had been dominated by food shortages and high taxes.
After outbreaks of violence on the streets of Paris, an armed mob surrounded Bastille on the morning of 14 July.
They stormed the prison and as word spread, the French Revolution was born.
The monarchy was eventually overthrown and Louis XVI and his wife Queen Marie Antoinette were executed.
How do French people celebrate?
The French flag and national anthem are symbolic of both the revolution and Bastille Day celebrations.
The Fete de la Federation, is the country's showpiece event.
It sees the oldest and largest military parade in Europe held on Paris's iconic Champs-Elysees.
Around 4,000 military personnel and police take part in the event which is also attended by the French president.
French people around the world, from Australia to the US, also hold events.
You will hear the words "liberty, equality and fraternity (brotherhood)" during the celebrations.
They were the three core beliefs of the people behind the revolution more than 200 years ago.
Freedom, fairness and togetherness remain at the heart of the French way of life today - and never more so than on 15 July, 2016.
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