Complex sentence: It commemorates the anniversary of the 1573-1574 Siege of Leiden, during the Eighty Years' War, when the Spanish attempted to capture the city, which was liberated from the Spanish Army in 1574.
Split into simpler sentences: It commemorates the anniversary of the Siege of Leiden, during the Eighty Years' War in 1573 and 1574, when the Spanish attempted to capture the city of Leiden. The city was liberated from the Spanish Army in 1574.