Tomorrow is Inauguration Day, a day of ceremonies to mark the peaceful transition of federal executive power within the United States government.

Here are five facts should know about the most important date (after Election Day) on the political calendar:

1. Inauguration Day used to be held on March 4. That was the original date (March 4, 1789) when the Confederation Congress, which operated under the Articles of Confederation, handed off power to the new constitutional government. When the new Constitution changed the opening day of Congress to the first Monday in December (Article I, Section 4), March 4 was chosen as the last day of the two-year legislative session. Because elections were held in November, this created an extended lame-duck session. The Twentieth Amendment changed the date of inauguration to noon on the 20th day of January.

2. The Constitution (Article II, Section 1, Clause 8) outlined the exact wording for the oath the president must take during the inauguration: “I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States.” No one knows for sure who began the tradition of adding on the end, “So help me God” (some claim it was George Washington, though there is pelling evidence to support that position). Since the Constitution doesn’t specify the wording of the oath for the Vice President,who is also the president of the U.S. Senate, he or she takes the same oath as senators.

3. George Washington started the practice of swearing-in with the ing president’s left hand on a Bible (which he borrowed from a Masons lodge). Washington also started the tradition of kissing the Bible after the oath. That tradition was followed by most every president until Dwight Eisenhower, who said a prayer instead of kissing the Bible. Theodore Roosevelt did not use a Bible and both John Quincy Adams and Franklin Pierce swore on a book of law. Lyndon B. Johnson was sworn in on a Catholic missal found on Air Force One even though he was a Protestant. Harry Truman, Dwight Eisenhower, Richard Nixon, and Barack Obama each swore the oath on two Bibles, as will Donald Trump.

4. Since 1937, every inauguration has included at least two prayers given by members of the clergy (Baptists have given a prayer 14 times; Catholics 12 times). The last official inaugural event also includes prayer: the National Prayer Service (January 21) at the Washington National Cathedral.

5. Following the inaugural ceremony at the U.S. Capitol, the outgoing President and First Lady leave to begin their post-presidential lives. As the Joint Congressional Committee on Inaugural Ceremonies notes:

In the early 20th century, a new tradition evolved whereby the outgoing president quietly left the Capitol immediately following the inaugural ceremony. In 1909, after congratulating President Howard Taft, former President Theodore Roosevelt left the Capitol for Union Station, where he took a train to his home in New York. In 1921, an ailing President Wilson panied President-elect Harding to the Capitol, but was too ill to remain during the ceremony. Outgoing Presidents Coolidge and Hoover also left the Capitol for Union Station where they traveled home by train. Outgoing Presidents Truman, Eisenhower, and Johnson left the Capitol by Car. Johnson and his family drove to Andrews Air Force Base where they boarded Air Force One for the trip home to Texas. . . . Since Gerald Ford’s departure in 1977, the former President and First Lady have left the Capitol grounds by helicopter (weather permitting).