April 17 1964
Ford Mustang was unveiled on the very first day of World Fair in New York.
Mustang was the brainchild of Lee Iacocca and his team of engineers. It was so well marketted that it became Ford's best seller then.
For more info:
First-generation Ford Mustang - Wikipedia, the free encyclopedia
PS: Just started this thread to keep track of automotive history timeline. bhpians please contribute, mods please keep track of descrepencies, ALso please move to proper heading, if this is out of place.