Full disclosure: I've never celebrated a "Constitution Day" in my life. I didn't even understand it was a holiday. I don't see the banks and post office closing, or anybody planning a parade (except a bunch of guys in kilts, and that's because our annual Highland Games are today - yay bagpipes!). If it weren't for the Internet, I wouldn't have known it was Constitution Day. But I'm all for people learning more about actual American history and foundations, so...
Robert Alt can get you started at No Left Turns, with his blog post with links.
Thomas Sowell asks Whose Constitution is it? - and demolishes the popular view that what matters is what the people who wrote it meant. (Hint: the document had to be ratified...)
The National Center for the Humanities has a Constitution Day page, with links.
And, if you're into behind-the-scenes stuff, here's Washington's Handwritten Annotations to the Constitution, as Dictated by the Committees.
Impressive Presentation Showing Sen. John Walsh's Plagiarism - It's remarkable when someone does the research to demonstrate extensive plagiarism from a public official [...]
56 minutes ago