I mean, being one of those unofficial "holidays" made up on the internet, I'd assume it's just that some of us want there to be a vore day, and who's going to stop us?
Specifically all at once, while alive, in a somehow sexualized way. I'm pretty sure there's a distinct difference between Vore and Cannibalistic fetishes.
Eight in Japanese is 八 which looks similar to katakana "ha" ハ and has the readings ハチ(hachi) ハツ(hatsu) や(ya) よう(you) Dates are usually written as 8月8日(8th month 8th day) though and the only onomatopoeia for stomach growling I can find is ごろごろ (gorogoro). Am I too stupid to get this? I can't see how the date 8/8 resembles the noise of a rumbling stomach
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.
I used to talk to a guy on IRC who was like one step above homeless, but he didn't want to get a real job, so he barely scraped by writing vore erotica on commission. If he wasn't such an asshole, I'd try to find out what happened to him and if he's still doing it because I know rule 34 and all, but are there more than like, I don't know, two people who are willing to pay some guy to write vore erotica for them on commission?