The National Calendar Day website said National Grouch Day has been celebrated since "at least 1976."
Oscar the Grouch got ready for his day a few weeks ago, tweeting that "Frowning makes me happy. Can't you tell?"
The Ornament Shop, on its website, listed its Top Five Grouches. Oscar won't be happy -- he only ranked No. 3. Topping the list was Ebenezer Scrooge, followed by the Grinch. Rounding out the top five are the Wicked Witch of the West and comedian Groucho Marx -- who wore a "grouch bag" around his neck that contained his valuables and money.
Here's an honorable mention: Jack Lemmon and Walter Matthau, who starred in the 1993 movie, "Grumpy Old Men." And Grumpy, one of the Seven Dwarfs.
You can commiserate with other grouches on Twitter by using the hashtag #NationalGrouchDay.
So, enjoy your day -- or, more in character -- be grouchy about it.