The date marks the anniversary of the historical 'Schuman declaration'. It takes place on 9 May every year to celebrate peace and unity in Europe.
At a speech in Paris in 1950, Robert Schuman, then France's foreign minister, set out his idea for a new form of political cooperation in Europe, which would supposedly reduce the risk of war between Europe's nations.
Fresh from the trauma of World War II, Schuman hoped his plan would ensure peace - particularly between France and Germany - in the future.
He proposed pooling the coal and steel production of different European countries and argued that this common venture would 'make it plain that any war between France and Germany becomes not merely unthinkable, but materially impossible.'
The European Steel and Coal Community created after Schuman's speech eventually grew into today's EU. The original six members (France, West Germany, Italy, the Netherlands, Belgium and Luxembourg) now number 28.
Europe day is one of a number of European symbols designed to foster unity among Europeans
To celebrate Europe day, the EU institutions opened their doors to the public on Sunday in Strasbourg, where Parliament meets this week (and Brussels later this month), while local EU offices in Europe and elsewhere will organise a variety of activities and events.