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I think that a holiday isn't a holiday unless it is a different day than the rest. You have to celebrate that day differently than you would a normal day. For example, Arbor Day is celebrated by planting trees and National Talk Like a Pirate Day is celebrated by talking like a pirate. Most people don't plant trees or talk like pirates every other day of the year, so these are actually holidays. However, "What if Cats and Dogs Had Opposable Thumbs" Day isn't a true holiday unless you celebrate that day differently than every other day. I doubt most people do. But, if you're wanting to make this a true holiday, maybe you could go "neener neener neener" in your dog's face all day while wiggling your own thumbs. Or you could use your opposable thumbs to stick tape on the bottom of your cat's feet. But, then PETA might get upset and then your holiday would be over, and you'd be back to just a "holiday."

Personally, I like "Eat Dessert Today because You didn't Yesterday" Day. This is a true holiday because you didn't celebrate it the day before because you didn't eat dessert. So, you don't eat dessert every day of the year, thus making it a holiday. You just get to celebrate the holiday every other day...