National Pennsylvania Day, celebrated on July 20 every year, honors the second state to join the Union. It is a day to commemorate the significance of Pennsylvania in American history, as well as its distinctive landmarks and culture that still exist today. Pennsylvania is also known as the Keystone State since it was once the temporary capital of the United States in Philadelphia. While the origin of the term has been lost, the meaning has not. Bridge builders understand that leaving the important keystone out of their construction would be a mistake that would result in collapse.