During the ongoing war against Sweden and before the Moscow campaign. The extraordinary sejm in Warsaw. 1659.
Sejm was convened at the beginning of year 1659 and the pre-sejm sejmiks were held in February of 1659. The sessions began on on 17 March and were finished, due to postponements, on 30 May 1659. Jan Gniński, starost [“starostwo” was a Polish term for eldership office] of Gniezno, was elected the marshal of the Chamber of Deputies. Lots of attention was given to approving the Treaty of Hadiach, signed with the Cossacks on 16 September 1658). Sejm adopted many resolutions concerning current affairs.
See: J. A. Chrapowicki, Diariusz. Część Pierwsza: lata 1656-1664 [The diary. Part one. 1658-1664 period.], edited by A. Rachuba, T. Wasilewski, Warszawa 1978, s. 189-194; S. Ochmann-Staniszewska, Z. Staniszewski, Sejm Rzeczypospolitej za panowania Jana Kazimierza Wazy. Prawo-doktryna-praktyka, [Sejms of the Commonwealth under the rule of John II Casimir Vasa. Law-doctrine-practice] T. 1, Wrocław 2000, s. 276-291. H. Olszewski, Sejm Rzeczypospolitej epoki oligarchii (1652-1763). Prawo-praktyka-teoria-programy [Sejm of the Commonwealth in the oligarchy era (1652-1763). Law-practice-theory-programs], Poznań 1966; Ustawodawstwo zob. Volumina Constitutionum, T. 4, cz. 2, Design and layout prepared by Stanisław Grodziski, Marcin Kwiecień, Krzysztof Fokt, Foreword Wacław Uruszczak, Warszawa 2017, s. 12-85.