I'm very confused if this item is broken or not, It's called placeholder item of mysterious something and its a chest, when I try to use it I can't. Is this on purpose? It came from the advent calendar.

The very name "Placeholder" suggests it will be replaced in a short period. I think its one of two things, either a new way to deal with not having an item set (earlier this month we just got errors). Or its an item that won't be available until the actual festival starts at Noon Sylestia time today.

I'm betting that it will change Noon today.