On this day, 215 years ago, Congress met in the Capitol Building for the first time. The Sixth Congress established the residence of the Congress and seat of the United States government in Washington, D.C. with the move on November 17, 1800.

On November 17, 1800, Congress moved to Washington from Philadelphia, convening in the newly completed north wing of the unfinished Capitol.
Picture from the Architect of the Capitol
The newly established United States had nine capitals between 1776 and 1800: Philadelphia, Baltimore, Lancaster, York, Princeton, Annapolis, Trenton, New York, and finally Washington, D.C. The U.S. Senate history includes a chronological table of the capitals and summarizes a book by Robert Forenbaugh called “The Nine Capitals of the United States.”

The Capitol in 1800 taken from advanced proof of plate 38 History of the United States Capitol by Glenn Brown Prints & Photographs Division, Library of Congress, //www.loc.gov/item/2014649261
By looking through our Statutes at Large collection, the first mention of the location and permanent seat of the United States Government was in the first Congress in 1790. Congress 1, Session 2, Chapter 28 is named, “An Act for establishing the temporary and permanent seat of the Government of the United States” (July 16, 1790). This act (later named the Residence Act or Permanent Seat of Government Act of 1790) established Philadelphia as the seat of government from 1790-1800 and then relocated Congress to Washington, D.C. in 1800. The Capitol was completed in 1826 and has since had extensions that have dramatically changed its physical appearance.
The Architect of the Capitol, Senate, House, and the Library of Congress have dedicated webpages to help researchers find more historical information.
And, of course, the Library of Congress was in the Capitol Building from 1800-1897!