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 Places of chusing Senators.
The Congress shall assemble at least once in every Year, and such Meeting shall [be on the first Monday in December]1, unless they shall by Law appoint a different Day.
1- This was changed by the 20th Amendment
Each state legislature gets to decide when to hold elections, except where Congress can override them. The start date for Congress has been changed by constitutional amendment in 1933 to the 3rd of January. For you trivia buffs, the 3rd of January also happens to be my wedding anniversary and my oldest daughters birthday.
Other Constitution posts.