Article Two of the United States Constitution
Clause 3 has to do with Senate recesses (times when the Senate is not meeting).  In the past, fast transportation was not available, and it could take a long time for members of Congress to get to Washington, D.C.  Because of this, Congress would usually meet only during planned "sessions," so everyone would have time to get there.  After the planned sessions were over, the Senate would go into recess, and all the Senators would go home.