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.
KRUSTY KRAB IS UNFAIR!
MR. KRABS IS IN THERE!
STANDING AT THE CONCESSION!
PLOTTING HIS OPPRESSION!