I looked it up and he was right:
Under the Article II of the Constitution of the United States ratified in 1789, the President has the power to make treaties--as long two-thirds of the Senate concurs--and to nominate ambassadors, public ministers, and consuls with the advice and consent of the Senate. These provisions placed the conduct of foreign affairs principally in the hands of the executive branch, but certain powers conferred upon the legislative branch--to declare war, appropriate funds, and advise and consent on treaties and appointments--gives the Congress significant ability to influence foreign policy. On May 19, 1789, then-Representative James Madison of New York introduced a bill to create an executive Department of Foreign Affairs headed by a Secretary of Foreign Affairs. On July 27, 1789, President Washington signed legislation to that effect. Congress passed another law giving certain additional domestic responsibilities to the new Department and changing its name to the Department of State and the name of head of the department to the Secretary of State. Washington approved this act on September 15, 1789. The domestic duties assigned to the Department of State were receipt, publication, distribution, and preservation of laws of the United States; custody of the Great Seal of the United States; authentication of copies and preparation of commissions of executive branch appointments; and, finally, custody of the books, papers, and records of the Continental Congress, including the Constitution itself and the Declaration of Independence.
Funny that no one makes a fuss about the date, or even observes it. I saw no Department Notice, no DipNote post, nothing except a lousy tweet, and not even an official tweet!