Constitution Day 2018 Constitution Day is the annual celebration of the day that representatives to the Constitutional Convention in Philadelphia completed and signed the U.S. Constitution in 1787. The observance of this day began as “I am an American Day” in 1940 and later as Citizenship Day in 1952 when the celebration was moved to September 17 to commemorate the signing of the original document. Constitution Day as we observe it today was recognized as a federal holiday in 2004,…