12 September 1683 is the date on which the Ottoman siege of Vienna ended.
In 1683, Vienna was struggling to survive a siege by the Ottoman Turks. A Pole named Kolscitzky, who was learned in Turkish, came to their rescue. He escaped through enemy lines to reach the Duke of Lorraine, who hurried to relieve the city. The Turks were repelled and Vienna was saved. Kolscitzky became very popular and famous. He persuaded a baker to produce a sweet bread roll in celebration of Vienna’s victory over the Turks. It was shaped like the crescent on the Turkish flag. We call them croissants because at some point the French took ownership of this Polish-Austrian idea.
Although croissant and crusade are similar words, they are not etymologically related, but there is a connection between them. While croissade-crusade came from Latin crux (French croix), croissant is French for crescent.
The crescent which the croissant imitates refers originally to the new moon as it grows towards the first quarter: the word comes from the Latin crescere to grow (from which we also get crescendo, and increase). As a new moon grows it is a waxing crescent moon (a tautology); after the first quarter it is waxing gibbous (from the Latin for hump) and then full. As the full moon declines, it is waning gibbous, then after the last quarter it is waning crescent (a contradiction in terms).
During his perilous journey, Kolscitzky had learned how to make coffee. After the siege ended, he came by a sack of coffee beans abandoned by the retreating Turks. He was the only person in Vienna who knew what coffee beans were for. He opened a café which quickly became famous for the drink and popular for its croissants. He served the coffee with milk and honey, a precursor of the style now known as Vienna coffee. Although the French stole the croissant, they had the good sense to leave Vienna coffee to the Viennese.