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.
Aha, I see, i have calculated this equation of where my enemy can and will go. The numbers are almost 85.1829% chance that they will go right over my crosshair, thus leading my equation into the correct mathematical path into my favor with the proceeding 1 kill or so. The simple equation to such a peasant game,
π·