Re: 19 June! Celebrate!
You are all incorrect.. a horrible travesty on this glorious day of pedantry: It's National Pedants' Day.
National Pedant's Day would be a day in honour of the National Pedant.
National Pedants Day would be a day concerning all of the pedants in the nation (possibly the one day they are in-season).
National Pedants' Day is a day in honour of all of the pedants in the Nation.