The history of 1815 actually begins before that with the Ursuline Order of Sisters.
They established a school and were short of teachers. Having come from France, they looked to France for more help. However, Pope Pius VII was, at the time, a prisoner of Napoleon.
Mother St. Michel know it was unlikely that the Pope would receive and authorize her request for help.
The letter did get through and the Mother did follow through with her promise. A statue was commissioned and placed in the convent chapel in 1810.
In 1815, was the Battle of New Orleans. American troops (6,000) under General Andrew Jackson faced off against British troops (15,000).
On the eve of the battle, New Orleans residents joined the Ursuline sisters at their convent. They prayed through the night. They prayed the rosary. The next morning, Mass was offered. The Prioress vowed to have a Mass of Thanksgiving annually if the Americans should win.
During the moment of communion at the Mass, a courier ran into the chapel and said the British had been defeated. They had been confused by a fog. An annual Mass of Thanksgiving has been held ever since.