Concept and creation
The numbers 5 and 9 can be read in Japanese as "go" and "ku", respectively, this is presumably why Akira Toriyama picked this date specifically.
In 2015, the real-world Japan Anniversary Association recognized on May 9 as Goku Day because of the aforementioned reason.
Additionally nine is considered to be unlucky because the word for nine (ku) sounds similar to the word for "pain/distress" (苦, kyū). This is likely why the dub changed the date of Piccolo Day to the 13th.
King Piccolo says that he will take a slip from the box (being held by his son, Piano) each year on the Piccolo Day, and the chosen sector on the slip will host that year's celebration and will end in its destruction at the hands of King Piccolo himself.
The ceremony is celebrated by King Piccolo once, in Age 753. The only sector chosen among the 43 sectors on Earth is sector 28, the location of West City. However, the city is not destroyed due to Goku killing King Piccolo on this day before the latter left Central City.
- May 9 'Officially' Recognized as Goku Day. Anime News Network (May 9, 2015).