The rational behind this use of the term is that if it is Opposite Day, then the correct way of proclaiming that it’s Opposite Day, is by actually proclaiming how it isn’t Opposite Day.
This is the most commonly used form of the phrase.2. A term that is used to genuinely describe how it is not opposite day. This use of the phrase is usually used by those that do not plan on partaking in Opposite Day, and therefor will mention that it’s actually “not Opposite Day” when people say it is (or on Opposite Day, isn’t). This usually does not work, as the pro-Opposite Day party acts as if the other party is in cahoots with them, not shutting them down.
2. “Stop saying that it’s opposite day, because it’s not Opposite Day.” … “That’s not what I meant, I’m not agreeing with you in Opposite language, I’m trying to tell you that it isn’t opposite day!” … “UGH!”