The date on which this piece is intended for first posting on this website,
March 4th, is the date on which- for 140 years, from George Washington's Second Inauguration in 1793 through Franklin Delano Roosevelt's First in 1933- Presidents of the United States, every four years until the 20th Amendment to the United States Constitution first took effect (36 times in all)- took the Oath of Office prescribed in Article II, Section 1, clause 8 of Our Nation's fundamental legal document.
The use of this date was not based on any specific constitutional language but was, rather, an accident of History: for the outgoing Congress of the Confederation that the Federal Government under the then-new Constitution would be replacing took it upon itself- soon after it had learned that at least 9 of the 13 original States of the American Republic had, by mid-1788, ratified that document (thereby putting it into effect, per its own terms)- to set, for the Year 1789, the dates for the first "appointment" of Presidential Electors (the first Wednesday in January); that on which said Electors would then meet separately, within their own States, to cast their respective Electoral Votes for President (the first Wednesday in February); and the earliest date on which the new Federal Government could thereafter begin operations (the first Wednesday in March: as a practical constitutional matter, this was the earliest both houses of Congress could first convene [for only Congress could then count and tabulate said Electoral Vote, and formally declare who had first been elected President, and Vice-President, of the United States- per the original Article II, Section 1, clause 3 of (since superseded by the 12th Amendment to) the Constitution, after which Congress could statutorily fill out other Executive offices and, in addition, provide for the Article III Judiciary, only once a President was by then in place to nominate- pending Senate confirmation- those who would thereafter serve in both the Executive and Judicial branches of this new Federal Government]).