The Utah Education Network has a TON of resources for Constitution Day linked on their site!
2012 marked the 225th anniversary of the signing of the Constitution!
In 2004, a federal law was passed that requires "each educational institution that receives Federal funds for a fiscal year shall hold an educational program on the United States Constitution on September 17 of such year for the students served by the educational institution".
Here's a Diigo list with links to resources on Constitution Day.
Teachinghistory.org has multiple primary sources and suggestions on how to use them in the classroom at their Spotlight: Constitution Day page.
ConstitutionDay.com offers online access to copies of the Constitution and amendments, as well as some brief biographies of the Founding Fathers who signed it.
iCivics is Supreme Court Justice Sandra Day O'Connor's effort to help increase civic competence in US students. It has a variety of interactive games that can be played to learn about our government, such as "Do I have a right?" and "Argument Wars".
The National Constitution Center has some ideas on how to observe Constitution Day at the elementary, middle, and high school levels.
Observing Constitution Day through the National Archives. The site includes a simulation of the Constitutional Convention, an online Constitution Workshop, and online exhibits of primary sources.
EdSiteMent! through the National Endowment for the Humanities has primary sources, information on the framers of the Constitution, educational games and interactive lessons, as well as tiered lessons for all grade levels.
Constitution Day resources from the Library of Congress include the American Memory Timeline, primary sources, and discussion questions you can use in the classroom.
The National Council for the Social Studies has a number of resources available for teaching the Constitution.
The Kansas State Department of Education's Educational Program Consultant for History, Government and Social Studies has a variety of resources for Constitution Day available on their agency website. Note: Kansas also mandates "Celebrate Freedom Week", and resources are also available for that on this page.
Information from the U.S. Senate on Constitution Day.
Educator and student resources from the National Constitution Center, including a constitutional timeline and an interactive Constitution. They have also introduced the Constitution Hall Pass, a free online video lesson on the Legislative Branch (2013). Or, play on the Pocket Constitution App!
Resources for Constitution Day from the Bill of Rights Institute. Play the Constitutional Duel or give your students the opportunity to be a reporter talking to James Madison!
Constitution Day information from the Center for Civic Education has lesson ideas for all grade levels.
What's your Constitution IQ? (From ConstitutionFacts.com)
The Georgia Department of Education's State Program Coordinator for Social Studies has a variety of resources for Constitution Day available on their agency website (scroll down about halfway).
Please note: All links to outside resources are free to public use. Any links those sites may have to resources for purchase are not reviewed or endorsed by the Wisconsin Department of Public Instruction.