The Japan Anniversary Association has accepted the request by film studio Toei to recognize May 9 as Goku Day. The specific date was selected because the numbers 5 and 9 pronounced together form the name of the main character Goku in Japanese.
Masako Nozawa, the voice of Goku, made her presence at a stage event, and shared moments from her career as a voice actress. Her professional preference was to work as a stage actress; she disliked voice acting at first. Still, she was successful in many voice acting auditions making the voice of young boys. At the audition for the role of Goku, Nozawa heard DB creator Toriyama decided her voice was the right one for the main character in the anime.
Because tomorrow is Mother’s Day, Nozawa also received a bouquet of 59 carnations from a staff member cosplayed as Goku who called her “the Mother of Dragon Ball.”
[Source]: Anime News Network: May 9 ‘Officially’ Recognized as Goku Day.