If i had to guess, what they're most likely refering to isn't onomatopoeia, but it's pronunciation. Vore in Japanese is 「丸呑み」(swallow whole) which is pronounced as まるのみ and sounds like 「丸のみ」(only circles) .

Using this as a rule for the date, the digits must be composed of only circles, i.e. either 0 or 8. This leaves only ８月８日 which fits the criteria.