Einstein's Happiest Thought

A common phrase often seen in textbooks is "Einstein's greatest blunder". Many Physics and Mathematics students are not aware of "Einstein's happiest thought". What was it?

In 1907 Albert Einstein was sitting in his patent office in Bern Switzerland when he had what he later described as “the happiest moment of my life”-that a person who falls freely under gravity feels weightless.
— From the Einstein Display at the Science Museum London, December 2015