Task: Question Understanding
Input (English): Passage: In May, the first movements of the war started as a Danish fleet under Jakob Brockenhuus sailed towards the Baltic. At Bornholm, on 30 May 1563, the fleet fired on the Swedish navy under Jakob Bagge, even though war had not officially been declared. A battle arose that ended with Danish defeat. German royal emissaries were sent to negotiate a peace, but at the meeting place of Rostock no Swedes appeared. On 13 August 1563, war was declared by emissaries from Denmark and Lübeck in Stockholm. The same month, Danish king Fredrik II attacked Älvsborg. At the beginning of the war the Danes advanced from Halland with a 25,000-strong army of professional mercenaries and captured Sweden's gateway to the west, Älvsborg Fortress, after only three days of bombardment and a six-hour assault on 4 September. This achieved the Danish aim of cutting off Sweden from the North Sea, blocking the all-important salt imports. Eric then attacked Halmstad, without result; the Swedish counterattack was driven back by the professional Danish army. After the king's departure from his army, Charley de Mornays stepped in as the commanding officer and was beaten by the Danish at the Battle of Mared. At sea a battle broke out near Öland on 11 September, whereafter the war took a pause.
Question: When did the Danish king Fredrik II attack Älvsborg?
Output (English): date