The original Constitution of the United States did not explicitly specify many dates, but one was the default date for which Congress was required to meet:
The Congress shall assemble at least once in every Year, and such Meeting shall be on the first Monday in December, unless they shall by Law appoint a different Day.
Why was this particular original date chosen?
Note that Clause 1 of the same section left the date of Congressional elections to be determined by state and federal laws, rather than an explicit date written into the Constitution (see this question). In turn, the date that was eventually chosen for these elections then influenced the writing of the 20th Amendment, which set a new default date for Congress to assemble (January 3rd). But how was the original date chosen?