Quoted from Wikipedia:
"In the 19th century, Hydra was home to some 125 boats and 10,000 sailors. The mansions of the sea captains that ring the harbor are a testament to the prosperity that shipping brought to the island, which, at the time of the Greek Revolution, had 16,000 inhabitants. During the revolution, the fleets of Hydra and the other two naval islands of Psara and Spetses were able to wrest control of the eastern Aegean Sea from the Ottoman Empire.
When the Greek War of Independence broke out, Hydra's contribution of some 150 ships, plus supplies, to fight against the Turks played a critical role. The Greek admiral Andreas Miaoulis, himself a settler on Hydra, used Hydriot fire ships to inflict heavy losses on the Ottoman fleet.
The Greek War of Independence, also known as the Greek Revolution (Greek: Ελληνική Επανάσταση, Elliniki Epanastasi; Ottoman: يونان عصياني Yunan İsyanı "Greek Uprising"), was a successful war of independence waged by the Greek revolutionaries between 1821 and 1832, with later assistance from Russia, the United Kingdom, France, and several other European powers against the Ottoman Empire, who were assisted by their vassals, the Eyalet of Egypt, and partly by the Beylik of Tunis.
Following the fall of the Byzantine Empire to the Ottoman Empire in 1453, most of Greece came under Ottoman rule. During this time, there were several revolt attempts by Greeks to gain independence from Ottoman control. In 1814, a secret organization called the Filiki Eteriawas founded with the aim of liberating Greece. The Filiki Eteria planned to launch revolts in the Peloponnese, the Danubian Principalities, and in Constantinople and its surrounding areas. The first of these revolts began on 6 March 1821 in the Danubian Principalities, but was soon put down by the Ottomans. The events in the north urged the Greeks in the Peloponnese into action and on 17 March 1821, the Maniots declared war on the Ottomans. This declaration was the start of a spring of revolutionary actions from other controlled states against the Ottoman Empire.
By the end of the month, the Peloponnese was in open revolt against the Turks and by October 1821, the Greeks under Theodoros Kolokotronis had captured Tripolitsa. The Peloponnesian revolt was quickly followed by revolts in Crete, Macedonia, and Central Greece, which would soon be suppressed. Meanwhile, the makeshift Greek navy was achieving success against the Ottoman navy in the Aegean Seaand prevented Ottoman reinforcements from arriving by sea.
Tensions soon developed among different Greek factions, leading to two consecutive civil wars. Meanwhile, the Ottoman Sultan negotiated with Mehmet Ali of Egypt, who agreed to send his son Ibrahim Pasha to Greece with an army to suppress the revolt in return for territorial gain. Ibrahim landed in the Peloponnese in February 1825 and had immediate success: by the end of 1825, most of the Peloponnese was under Egyptian control, and the city of Missolonghi—put under siege by the Turks since April 1825—fell in April 1826. Although Ibrahim was defeated in Mani, he had succeeded in suppressing most of the revolt in the Peloponnese and Athens had been retaken.
Following years of negotiation, three Great Powers, Russia, Britain and France, decided to intervene in the conflict and each nation sent a navy to Greece.
Following news that combined Ottoman–Egyptian fleets were going to attack the Greek island of Hydra, the allied fleet intercepted the Ottoman–Egyptian fleet at Navarino.
Following a week-long standoff, a battle began which resulted in the destruction of the Ottoman–Egyptian fleet, which is celebrated annually in Hydra during the Miaoulis Festival week in June. With the help of a French expeditionary force, the Greeks drove the Turks out of the Peloponnese and proceeded to the captured part of Central Greece by 1828. As a result of years of negotiation, Greece was finally recognized as an independent nation in May 1832.
The Revolution is celebrated by the modern Greek state as a national day on 25 March."
Because the 25th March (regardless of what day of the week it is) is a national day, the entire civil, banking and postal service has a holiday and many shops are closed execpt the tourist shops in Hydra.