25 Pa. Stat. § 2779
Candidates to fill vacancies in the offices of Representative in Congress, Senator and Representative in the General Assembly and member of the council or legislative body of any city, borough, town or township shall be nominated by political parties, in accordance with the party rules relating to the filling of vacancies, by means of nomination certificates, in the form prescribed in section 630 of this act; and by political bodies, by means of nomination papers, in accordance with the provisions of sections 951, 952 and 954 of this act. Said nomination certificates and nomination papers for the office of Representative in Congress, Senator and Representative in the General Assembly shall be filed in the office of the Secretary of the Commonwealth not later than fifty (50) days prior to the date of the special election, and for the office of member of the council or legislative body of a city, borough, town or township, in the office of the county board of elections wherein such city, borough, town or township is situate, not later than fifteen (15) days after the issuance of the writ of election.
25 P.S. § 2779