The Venetian position in the Peloponnese could not be secure, however, as long as the Ottomans held onto eastern Central Greece, where Thebes and Negroponte  were significant military strongholds. Thus, on 21 September 1687, Königsmarck's army, 10,750 men strong, landed at Eleusis, while the Venetian fleet entered Piraeus. The Turks quickly evacuated the town of Athens, but the garrison and much of the population withdrew to the ancient Acropolis of Athens, determined to hold out until reinforcements arrived from Thebes. The Venetian army set up cannon and mortar batteries on the Pnyx and other heights around the city and began a siege of the Acropolis, which would last six days  and would cause much destruction to the ancient monuments. The Ottomans first demolished the Temple of Athena Nike to erect a cannon battery, and on 25 September, a Venetian cannonball exploded a powder magazine in the Propylaea. The most important damage caused was the destruction of the Parthenon. The Turks used the temple for ammunition storage, and when, on the evening of 26 September 1687, a mortar shell hit the building, the resulting explosion killed 300 people and led to the complete destruction of the temple's roof and most of the walls. Despite the enormous destruction caused by the "miraculous shot", as Morosini called it, the Turks continued to defend the fort until a relief attempt from the Ottoman army from Thebes was repulsed by Königsmarck on 28 September. The garrison then capitulated, on condition of being transported to Smyrna.
Question: where Thebes and Negroponte were significant military strongholds?
Answer: 21 September 1687