Article I Section 4

The Times, Places and Manner of holding Elections for Senators and
Representatives, shall be prescribed in each State by the Legislature
thereof; but the Congress may at any time by Law make or alter such
Regulations, except as to the Place of Chusing Senators. 

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.