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.
๐๐ป๐ธ๐ฝ๐ฑ๐ฎ๐ป ๐ ๐ป๐ฎ๐ฌ๐ฎ๐ฒ๐ฟ๐ฎ๐ญ ๐๐ธ๐พ๐ป ๐ถ๐ฎ๐ผ๐ผ๐ช๐ฐ๐ฎ ๐๐ฎ๐ต๐ต. ๐๐ฝ ๐ฎ๐๐ฌ๐ฒ๐ฝ๐ฎ๐ผ ๐ถ๐ฎ ๐ฝ๐ธ ๐ฒ๐ท๐ฏ๐ธ๐ป๐ถ ๐๐ธ๐พ ๐ช๐ซ๐ธ๐พ๐ฝ ๐ช ๐ฌ๐ธ๐ฌ๐ด ๐ป๐ฎ๐ผ๐ฎ๐ป๐ฟ๐ฎ, ๐ฏ๐ป๐ฎ๐ฝ ๐ท๐ธ๐ฝ, ๐ฌ๐ธ๐ฌ๐ด ๐๐ฒ๐ต๐ต ๐ซ๐ฎ ๐ผ๐ฎ๐ท๐ฝ ๐๐ธ๐พ๐ป ๐๐ช๐ ๐ผ๐ธ๐ธ๐ท.