The story
A Mexican army of about four thousand, badly equipped, held off a French force twice its size at Puebla. It did not end the war — the French took Mexico City the next year — but it was the first proof that they could be held.
It is not Mexican independence day, which is 16 September, and it is a minor date in most of Mexico. It became a large festival in the United States instead, first among Mexican-Americans in California and then generally.