June 6th is widely celebrated as National Yo-yo Day, as it is the birthday of Donald Duncan SR, the founder of Duncan Toys. Here is a bit of history about the man himself…
Donald F. Duncan, Sr. (June 6, 1892 –May 15, 1971) was an American entrepreneur and inventor, and founder of the Duncan Toys Company. Donald Duncan is most commonly associated with the Yo-Yo, the commercial success and iconic status they enjoyed during the 20th century in the United States and the world being largely the result of his marketing efforts.
Duncan is often miscredited with invention of the Yo-Yo. The name Yo-Yo was a trademark of his company from 1930 until 1965, when the case Donald F. Duncan, Inc. v. Royal Tops Mfg. Co., 343 F.2d 655 (7th Cir. 1965) resulted in a federal court of appeals ruling in favor of the Royal Tops Company, asserting that (the trademark had become a part of common speech). This ruling has been widely criticized  as a miscarriage of justice by legal historians. If used as a precedent, it would imperil many other similarly successful trademarks such as Frisbee Slinky and Rollerblade.
Among the other companies he founded are the Good Humor mobile frozen treats franchise and a parking meter manufacturing company. Most notable among the innovations credited to Duncan is the concept of the premium incentive—a marketing tactic wherein the consumer is encouraged to collect proofs of purchase and redeem them for rewards, such as small toys or discount coupons.
He died in an automobile accident.