Congress moved to Washington, D.C. on this day in 1800 (or tomorrow, depending on your source). The Capitol was yet unfinished, but the legislators decided to meet in the completed north wing of the building. For the prior decade, Philadelphia had hosted the U.S. Congress as the nation's capital. Previous iterations of Congress going back to the Revolutionary War had been seated in nine different locations. The Capitol wouldn't be completed until 1826.
This new sixth U.S. Congress was comprised of 32 senators, 106 representatives, and one non-voting delegate from the Northwest Territory (at least at first, until it was divided into two parts and later admitted piece by piece as states). It would be the last Congress dominated by the Federalist party, which favored industrialization and a strong federal government, before the more agrarian Democratic Republican party began to run the table in elections for years to come.
You can read more about the history of the Capitol building at its website.
