2 U.S.C. § 198

(a) Unless otherwise provided by the Congress, the two Houses shall— (1) adjourn sine die not later than July 31 of each year; or (2) in the case of an odd-numbered year, provide, not later than July 31 of such year, by concurrent resolution adopted in each House by roll-call vote, for the adjournment of the two Houses from that Friday in August which occurs at least thirty days before the first Monday in September (Labor Day) of such year to the second day after Labor Day. (b) This section shall not be applicable in any year if on July 31 of such year a state of war exists pursuant to a declaration of war by the Congress.