The Doodle doesn't commemorate a specific date, merely the 40th year since the toy's invention. But the specific number encoded in the day-month combination -- 5/19 -- is shorthand for the 519 quintillion potential cube configurations.
Google had long wanted to implement a Rubik's Cube Doodle, but the existing complement of web technologies was never yet sufficient to convey the desired experience. Not until now! The missing ingredient was CSS 3-D Transforms.