This holiday is always celebrated on 1 December and marks the reinstatement of sovereignty after the period of Spanish rule between 1580 and 1640.
Portugal was plunged into a a royal crisis in 1587 following the death of the young King D. Sebastião at the battle of Alcácer Quibir in North Africa.
The King of Spain, Philip II, seized this regal vacuum and invoked his right to succession - he was the grandson of D. Manuel I of Portugal. He ascended to the throne and managed to achieve an Iberian Union for the first and only time.
With Philip II as King, Portugal became involved in a war with the traditional enemies of Spain, such as England and the Low Countries. Portugal lost a great part of its naval fleet and much of it's empire - though they did manage to thwart Dutch ambitions in Africa and South America.
Fearing for the loss of Portugal's status, a revolution was led by the nobility and high bourgeoisie. On 1 December 1640, 60 years after the crowning of Philip I, the Portuguese monarchy was restored and and a new ruling dynasty, the House of Bragança, was founded.