The most important rule of Unicorn Day is to have fun, fun, fun! The unicorns are celebrating their favorite day of the year, complete with rainbows and butterflies. But when it’s revealed there’s an impostor in their midst (a horse with a fake horn), the party comes to a screeching halt. But it turns out Unicorn Day is for all friends! The party continues in all its glittery glory, but this time everyone is invited, fake horns and all.