Growing grass over grass...
That is, "using '草' together with 'w'", which is considered to be redundant.
The 3rd of September is
Grass Day!
Or I guess you could say 'lmao day' (see below).
Either way, the date is a pun:
Grass → kusa → ku(9) sa(3)
The use of the kanji for grass (草) as a way to represent laughter came from the shortening of 笑 (the kanji for laughter, read as warau) to "w". Whenever several of those are lined up together, such as "www", it looks like a tuft of grass, and that's apparently why they started to refer to it as such!
It's bad manners!That's so 草w
I guess the English equivalent would be lmaololBy the way, it seems like overseas, the use of "lol" as a big laughter has gotten somewhat old. I reckon they use stuff like "haha" or "hehe" nowadays.