You can only collect these chests during the Halloween period (pc clock between Oct 29th-Nov 2nd) or during the Bitterleaf season in game.

Located in wolf cave in a secret area to the top right. Break the wall with some kind of club or mace. Also the same location Moth Princess gets a unique look if you bring her here.
Unlocks Outfit Beaming Bride

Located in the Goddess Monastery in the back area by the cathedral garden, top right by the gallows

Unlocks outfit Final Plea

Located at the Holstaur Farm in the back area where the cow races start.

Unlocks the outfit Cow Bones.

[BETA CONTENT ONLY] You can only access the area the outfit is in by opting into the beta, and risking all bugs that comes with that, if you don't wanna risk that I suggest waiting till these areas are out of beta.
Mermaid Reign is finally out of beta, yay!

Located in the Bilge in the mermaid's reign, currently the only way I know to access the bilge is to let the reign fall so the guard leaves, if you know another method let me know.

Unlocks outfit Deep Creature

Located in the Kobold Mines Near the back where you do the quest Crystal Fever.

Unlocks the outfit Cucco Knight.

Kalamata For Crusader Princess and Amelie Locations.