March 4, 1789 was the date set for the new federal government to begin operating under the ratified Constitution. In practice, the first Congress struggled to assemble a quorum in New York City, and George Washington would not be inaugurated until April 30. Still, the date marks the formal birth of the constitutional government that has governed the United States ever since.
The Constitution had been ratified by the required nine states by mid-1788, but turning parchment into a functioning government proved slower than anyone expected. Congress designated March 4, 1789 as the day the new government would commence, and representatives began drifting toward New York's Federal Hall—by horse, by boat, over muddy late-winter roads.
They drifted rather too slowly. The House of Representatives didn't achieve a quorum until April 1, and the Senate followed five days later. George Washington wasn't inaugurated until April 30, nearly two months after the Constitution's official start date. The anticlimax became a minor embarrassment for a government eager to project stability and legitimacy to skeptical Europeans and skeptical Americans alike.
Despite the shaky start, March 4 stuck as a symbolic anchor. It later became the standard date for presidential inaugurations for over a century, until the Twentieth Amendment moved the date to January 20 in 1933, partly to shorten the awkward lame-duck period this very same sluggish transition had first revealed.