No, seriously, you could look it up. It's not just college kids hijacking Wikipedia, it's a real, by-gosh day established in 1992 by Briton Mary Evans Young, Director of the group Diet Breakers.
Wikipedia says that today the day to celebrate "body acceptance and body shape diversity," a day "dedicated to promoting a healthy life style and raise awareness of the dangers and futility of dieting."
So... Happy International No Diet Day. See you at brunch.