According to their calender the session ends Friday May 17, 2013 at 6:00 P.M.
The wording on the calender is confusing. May 17 this year is the first Friday after the second Monday in May, which is the last day the General Assembly can consider bills (effectively the end of the session for bills). However, the session is not officially over until May 30.
From Article III, Section 20 of the Missouri Constitution:
Regular sessions of assembly--quorum--compulsory attendance--public sessions--limitation on power to adjourn.
Section 20. The general assembly shall meet on the first Wednesday after the first Monday in January following each general election. The general assembly may provide by law for the introduction of bills during the period between the first day of December and the first Wednesday after the first Monday of January.
The general assembly shall reconvene on the first Wednesday after the first Monday of January after adjournment at midnight on May thirtieth of the preceding year. A majority of the elected members of each house shall constitute a quorum to do business, but a smaller number may adjourn from day to day, and may compel the attendance of absent members in such manner and under such penalties as each house may provide. The sessions of each house shall be held with open doors, except in cases which may require secrecy but not including the final vote on bills, resolutions and confirmations. Neither house shall, without the consent of the other, adjourn for more than ten days at any one time, nor to any other place than that in which the two houses may be sitting.
Section 20(a).
The general assembly shall adjourn at midnight on May thirtieth until the first Wednesday after the first Monday of January of the following year, unless it has adjourned prior thereto. All bills in either house remaining on the calendar after 6:00 p.m. on the first Friday following the second Monday in May are tabled.
The period between the first Friday following the second Monday in May and May thirtieth shall be devoted to the enrolling, engrossing, and the signing in open session by officers of the respective houses of bills passed prior to 6:00 p.m. on the first Friday following the second Monday in May.
The general assembly shall automatically stand adjourned sine die at 6:00 p.m. on the sixtieth calendar day after the date of its convening in special session unless it has adjourned sine die prior thereto.