The bulk update request #6494 is pending approval.
create alias pokemon-specific_day (354) -> pokemon (515690)
create alias eevee_day (9) -> eevee (17269)
create alias skitty_day (3) -> skitty (1035)
create alias sandshrew_day (3) -> sandshrew (557)
create alias sandslash_day (2) -> sandslash (1140)
create alias jolteon_day (13) -> jolteon (6984)
create alias milotic_day (14) -> milotic (1721)
create alias gardevoir_day (8) -> gardevoir (19422)
create alias glaceon_day (11) -> glaceon (11881)
create alias mewtwo_day (12) -> mewtwo (5587)
Reason: pokemon-specific_day was added to over 300 posts apparent by a single user. I see no reason for this tag as it says nothing about the contents of the image, it's just matching certain pokemon to certain days; it doesn't even say which pokemon.
As I understand it, these "<pokemon> Day"s started as a phonetic pun because the way certain dates are or can be said in Japanese sound like certain pokemon names. Beyond that, these aren't really special, it's just drawing or posting previously made drawings of particular pokemon on a particular day (and I have a suspicion some of them aren't even pun-based, it's just an arbitrary day) for appreciation. The Zangoose Day tag was allowed to stay because it had a visible emblem, which I remember there being discussion saying that the emblem should be visible for the tag to apply (though this doesn't seem to have been mentioned in the wiki, and a number of posts with the tag don't have the emblem; that tag should probably be revisited, but I won't touch it here since it ostensibly had been valid). None of these other days have such an emblem or other visible mark, they're just being applied because it's mentioned somewhere (including post descriptions).
The list of days isn't exhaustive. These are just the ones I've happened to noticed. I'm also not sure if aliasing to specific pokemon is the best idea, but it's almost certainly likely a post appreciating a particular pokemon will have that pokemon. Alternatively I can alias to just pokemon instead.
Updated