>>46394238Can't really say for sure the specifics as I'm not Japanese myself, but certain Pokemon have specific days where artists go to draw art of that Pokemon (mostly a pixiv thing). Zangoose day is probably their biggest one and contributes a very large portion of the mon's art.
I think it's usually to do with a pun regarding the Pokemon's name/dex number and the date - everyone knows nips love their puns.