U.S. Constitution, Article I — The Legislative Branch, 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.
Notes
Requires Congress to assemble at least once in every year, and sets the meeting for the first Monday in December unless Congress, by law, appoints a different day. The annual meeting is mandatory, while its date can be changed by an ordinary act of Congress.
Use as reference
Lexinter.net | U.S. Const. art. I, § 4, cl. 2 | https://laws.lexinter.net/constitution/us/article-1/section-4/clause-2