It can never be opposite day. When ever you say "HEY IT'S OPPOSITE DAY!" the opposite would apply therefor it would not be opposite day. Also if you say "IT'S NOT OPPOSITE DAY!" the affect is not applied.
𝓜𝔂 𝓭𝓮𝓪𝓻𝓮𝓼𝓽 𝓫𝓻𝓸𝓽𝓱𝓮𝓻, 𝓽𝓸𝓭𝓪𝔂 𝓶𝓪𝓻𝓴𝓼 𝓽𝓱𝓮 8𝓽𝓱 𝔂𝓮𝓪𝓻 𝔀𝓮 𝓱𝓪𝓿𝓮 𝓱𝓪𝓭 𝔀𝓲𝓽𝓱𝓸𝓾𝓽 𝓿𝓪𝓻𝓲𝓮𝓽𝔂. 𝓦𝓮 𝓶𝓪𝔂 𝓷𝓸𝓽 𝓫𝓮 𝓪𝓫𝓵𝓮 𝓽𝓸 𝓮𝓷𝓭𝓾𝓻𝓮. 𝓟𝓵𝓮𝓪𝓼𝓮 𝓹𝓻𝓪𝔂 𝓯𝓸𝓻 𝓸𝓾𝓻 𝓼𝓪𝓵𝓿𝓪𝓽𝓲𝓸𝓷. .