Edit: thanks to taneliv for noticing it works both ways! (original post only showed one way)
Next stop: world peace :P
This is not an ISO 8601 or RFC 3339 date.
So the entire world celebrates Labour Day based on an event in the US, while the US alone uses a different date. And somehow that doesn't even seem that unusual to me; the US often is the first with a new idea, and as soon as others join in, they back down.
As I said in my post, the US aren't alone in that, Japan also uses a different date. Canada also celebrates it on a different day, the same as the US. Ireland celebrates it on the first Monday in May.