Article 1, Section 4, Clause 2

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.

1.William Blackstone, Commentaries 1:144--49, 1765
2.Declaration of Independence, 4 July 1776
3.Records of the Federal Convention
4.Joseph Story, Commentaries on the Constitution 2:§§ 827--28 (1833)