Schools that receive federal funding are required to "hold an educational program pertaining to the United States Constitution".1 Meet your Constitution Day teaching requirement with fun, engaging, and easy-to-use games and lesson plans from iCivics:
In Do I Have a Right?, students run their own law firm and help clients resolve funny yet thought-provoking issues. They'll learn about the Bill of Rights and other amendments to the Constitution. Check out our Game Guide for Teachers to learn how to use this game in your classroom.
This free Constitution Day lesson plan provides a great overview of the Constitution and how Articles I-III describe the structure, function and powers of our three branches of government.

1 For all you hard-core civics geeks, take a look at the notice in the Federal Register on implementation of Constitution Day.